Shi'ite deputy speaker of the Iraqi Council of Representatives Haider Abadi, a member of Iraqi Prime Minister Nuri al-Maliki's State of Law bloc, addresses a news conference in Baghdad in this July 15, 2014 file photo.[Photo/Agencies]

BAGHDAD - Iraqi President Fuad Masoum accepted Monday the nomination of Haidar al-Abadi as prime minister-designate, making a significant progress in the country's troubled political process, but the nomination angered the outgoing Prime Minister Nuri al-Maliki who threatened to sue his opponents.

Abadi's nomination shaped a major breakthrough in the country's political process which faces a great challenge since early in June when the Sunni militant groups, including the extremist Islamic State group, an al-Qaida offshoot, took control of Iraq's second largest city of Mosul and later seized swathes of territories after Iraqi security forces withdrew from their posts in Nineveh and other predominantly Sunni provinces.

The state-run Iraqiya channel showed Masoum signing the letter, granting Abadi, the Shiite coalition's nominee for prime minister, the power to form the next government.

The Iraqi Speaker Salim al-Jubouri and Ibrahim al-Jaafari, the head of the Iraqi National Alliance, were present at the televised meeting.

Abadi thanked Masoum and pledged to do his best in the coming time within one month, a time span stipulated in the constitution.

He called on all Iraqis to unite in fighting against the extremist Islamic State militants which swept large areas in northern and western the country.

"We all have to cooperate to fight back the terrorist campaign waged on Iraq and to stop all terrorist groups," he said during the ceremony just after Masoum asked him to form a cabinet.

Abadi, 62, is the head of political office for the Islamic Dawa Party, headed by the outgoing Prime Minister Nuri al-Maliki. He became a member of the party in 1967 and a member of its executive leadership in 1979. Saddam Hussein's Baath regime executed two of his brothers and imprisoned a third for ten years. He lived in exile in London during the reign of Saddam Hussein.

He was appointed minister of communications in the Iraqi Governing Council in 2003, and was elected member of Iraqi Parliament in 2005. Later, he chaired the parliamentary committee for economy, investment and reconstruction. In 2010 he was re- elected as a parliament member, and in 2013, he chaired the finance parliamentary committee.

Earlier, the Shiite lawmakers in the umbrella organization the Iraqi National Alliance, which includes Maliki's State of the Law bloc, sent a letter signed by 127 lawmakers, out of more than 170 members in the alliance, to the Iraqi President Masoum, nominating Abadi to replace Maliki.

On July 24, Iraqi lawmakers elected Masoum as the new president, marking an important step toward in forming a new government in the violence-torn country.

Within two weeks after the new president is elected, he must ask the "largest bloc" in the parliament to nominate a prime minister to form a new government, according to Iraq's constitution.

During the day, the state-run Iraqiya channel reported that the Iraqi federal court ruled Maliki's bloc is the largest one in parliament, giving an impression that such a rule would pave the way for Maliki to stay on as prime minister.

Later on, the court said on its official website that its rule was a repetition to its earlier interpretation to Article 76 of the Iraqi constitution, which mentioned the expression of largest bloc without further specification.

Late on Sunday, Maliki said that he would file a legal complaint against Masoum for violating the constitution by refraining from asking his State of Law Coalition to form the next cabinet after the end of the 15-day constitutional timing.

Following Maliki's tough speech, Iraqi security forces intensified measures across the Iraqi capital of Baghdad. The special forces loyal to Maliki were deployed in strategic areas in Baghdad, in particular the areas surrounding the Green Zone, which houses most Iraqi top offices and ministries, as well as the U.S. embassy.

The deployment of the special forces would send a signal to Maliki's political rivals that "he is not willing to retreat from seeking a third-term in office," Jamaa Diwan, a lawmaker from Ahrar parliamentary bloc, loyal to Shiite cleric Moqtada al-Sadr, told reporters.

Amid the political and security tensions in the Iraqi capital, the UN special envoy to Iraq welcomed the decision of President Masoum to accept the nomination of Haidar al-Abadi as prime minister-designate, confirming that the security forces should not intervene in political process.

"The President of the Republic has fulfilled his role in accordance with the provisions of the Constitution and asked the nominee of the largest political bloc to form a government," Nickolay Mladenov, the Special Representative of the UN Secretary- General for Iraq said in a statement posted on the Internet.

Mladenov, also head of the UN Assistance Mission for Iraq ( UNAMI), stressed on the significance for all political blocs in the parliament to cooperate in forming an "inclusive government" that reflects the wishes of the Iraqi people.

He also urged the political leaders to show "moderation" in their statements and actions, confirming that the "security forces should refrain from actions that may be seen as interference in matters related to the democratic transfer of political authority. "

However, the defiant Maliki would not easily give up despite the fact that he has been under growing pressure to abandon his attempt for a third term, as the National Alliance, a Shiite bloc, which includes Maliki's State of the Law, sees that the next prime minister must be accepted by all political factions, including the Shiites, the Kurds and Sunnis.

In the afternoon, a group of politicians allied to Maliki said in a statement read on Iraqi official television that Maliki's Islamic Dawa Party rejected the nomination of the new Prime Minister Haidar al-Abadi, saying that he is not representing them and has no legitimacy.

"Haidar al-Abadi is only representing himself and does not represent the State of the Law Coalition," Khalaf Abdul Samad, a member of Maliki's Dawa Party, said in the statement.

"We reserve our right to file a lawsuit against whoever violated the constitution," Samad said with presence of Maliki himself.

A few hours later, angry Maliki appeared again on the Iraqiya channel and pledged to fix what he considered "mistake" that happened when a new prime minister was nominated, charging the nomination was a dangerous violation to the country's constitution.

The mistake is a setback which will be fixed as the political process is heading the right direction, Maliki said in a televised speech.

Maliki confirmed that his bloc has filed a lawsuit to the court and his bloc has all the needed evidences to prove that the State of the Law coalition is the largest bloc in the first session of the parliament, according to the constitution.

"The external collaboration was disclosed when we rejected the constitutional violation, the American Administration stood at the side of those who violated the constitution," Maliki said, accusing his political rivals of collaborating with the United States to confiscate the elections results.

Maliki's remarks came after the U.S. Vice President Joe Biden phoned Haidar al-Abadi and conveyed the Obama administration's congratulations to Iraq's new leadership. The two leaders discussed implementation of the bilateral Strategic Framework Agreement.