Russian and U.S. negotiators discussed the expiration of the last remaining nuclear arms pact between the two countries and agreed on the need to quickly launch new arms control talks, the Kremlin said Friday.

The New START treaty terminated Thursday, leaving no caps on the two largest atomic arsenals for the first time in more than a half-century and fueling fears of an unconstrained nuclear arms race.

The United States on Friday urged three-way talks with Russia and China to set new limits on nuclear weapons, after the last treaty between top nuclear powers Washington and Moscow expired.

Thomas DiNanno, the under secretary of state for arms control, told the Conference on Disarmament in Geneva that "serial Russian violations, growth of more worldwide stockpiles and flaws in New Start's design and implementation gives the United States a clear imperative to call for a new architecture that addresses the threats of today, not those of a bygone era".

The Kremlin said Friday that two days of talks with Ukraine, mediated by the United States, on how to find a possible end to the war were difficult but "constructive", saying that they will continue.

"For two days there was constructive and very difficult work," Kremlin spokesman Dmitry Peskov told reporters a day after the Abu Dhabi negotiations ended, adding: "they will continue."

Iran and the United States will hold talks Friday in Oman, their latest over Tehran's nuclear program after Israel launched a 12-day war on the country in June and the Islamic Republic launched a bloody crackdown on nationwide protests.

U.S. President Donald Trump has kept up pressure on Iran, suggesting America could attack Iran over the killing of peaceful demonstrators or if Tehran launches mass executions over the protests. Meanwhile, Trump has pushed Iran's nuclear program back into the frame as well after the June war disrupted five rounds of talks held in Rome and Muscat, Oman, last year.

The U.S. and Russia agreed on Thursday to reestablish high level military-to-military dialogue following a meeting between senior Russian and American military officials in Abu Dhabi, the United States European Command said in a statement.

The agreement was reached following meetings between Gen. Alexus Grynkewich, the Commander of U.S. European Command -- who is also NATO Supreme Allied Commander Europe -- and senior Russian and Ukrainian military officials, the statement said.

Keir Starmer vowed Thursday to remain Britain's prime minister despite a storm over his appointment last year of disgraced politician Peter Mandelson, linked to late U.S. sex offender Jeffrey Epstein, as Britain's envoy to Washington.

"I intend to go on doing that vital work for our country because I believe it is the absolute focus and of first importance for this government," he told reporters, after apologizing to the victims of Epstein for believing Mandelson's "lies" about their ties.

Ukraine and Russia have conducted their first prisoner exchange in months, each releasing 157 captured soldiers, Russia's defense ministry said Thursday, amid U.S.-brokered talks in Abu Dhabi aimed at ending the war.

"On February 5, 157 Russian servicemen were returned" from Ukraine, the defence ministry said in a statement. It added that "in exchange, 157 prisoners of war of the Ukrainian Armed Forces were handed over."

President Donald Trump said Wednesday that he and Chinese President Xi Jinping discussed the situation in Iran in a wide-ranging call as the U.S. administration pushes Beijing and others to further isolate Tehran.

Trump said the two leaders also discussed a broad range of other critical issues in the U.S.-China relationship, including trade and Taiwan and his plans to visit Beijing in April.

China said on Thursday it would not join nuclear talks "at this stage" after the expiry of the U.S.-Russian START treaty triggered fears of a global arms race.

"China's nuclear capabilities are of a totally different scale as those of the United States and Russia (and) will not participate in nuclear disarmament negotiations at this stage," Beijing's foreign ministry spokesman Lin Jian told a news conference.

Russia said Thursday there had been "progress" in talks with Ukraine in Abu Dhabi on finding an end to the four-year-war sparked by Moscow's full-scale invasion of its pro-Western neighbor.

"There is definitely progress, things are moving forward in a good, positive direction," Russian negotiator Kirill Dmitriev told state media.