WASHINGTON — Left-wing US lawmakers on Monday urged President Joe Biden to seek a negotiated settlement with Russia to end the Ukraine conflict, including the exploring of security arrangements acceptable to both sides.

In a letter, 30 House members from Biden's Democratic Party said they agreed with the White House that a settlement was up to Kyiv.

But as legislators responsible for the expenditure of tens of billions of US taxpayer dollars in military assistance in the conflict, such involvement in this conflict also creates a responsibility for the United States "to seriously explore all possible avenues", said the lawmakers led by Representative Pramila Jayapal, leader of the House Progressive Caucus.

They called for direct engagement with Russia.

"Such a framework would presumably include incentives to end hostilities, including some form of sanctions relief, and bring together the international community to establish security guarantees for a free and independent Ukraine that are acceptable for all parties, particularly Ukrainians," they wrote.

When asked about the letter, State Department spokesman Ned Price said nobody wants to see this conflict end more than their Ukrainian counterparts.

"We don't know when it will transpire, principally and solely because we have not seen any indication from the Russians that they are prepared to engage in that diplomacy and dialogue," he told reporters.

White House press secretary Karine Jean-Pierre said separately: "We've been very clear: nothing about Ukraine without Ukraine."

Top Russian officials phoned Western counterparts on Sunday and Monday to tell them Moscow suspected Ukraine of planning to use a so-called dirty bomb laced with nuclear material, Reuters reported.

Ukrainian President Volodymyr Zelensky said Russia planned such an attack itself to blame Ukraine.

Russia sent a letter on its claims about Kyiv to the United Nations on Monday and was expected to raise the issue with the Security Council at a meeting on Tuesday.

"We will regard the use of the dirty bomb by the Kyiv regime as an act of nuclear terrorism," Russia's UN Ambassador Vassily Nebenzia told UN Secretary-General Antonio Guterres and the Security Council in the letter, seen by Reuters.

Also, the UN nuclear watchdog is preparing to send inspectors in the coming days to two Ukrainian sites at Kyiv's request over the dirty bomb issue, it said on Monday.

German President Frank-Walter Steinmeier vowed further support to Ukraine during a surprise visit to Kyiv on Tuesday.

Steinmeier had originally planned to visit Ukraine in April, but Kyiv refused to welcome him then. Kyiv and Berlin later patched up their disagreement.

Agencies via Xinhua