U.S. Secretary of State Antony Blinken said Thursday he spoke briefly with Russian Foreign Minister Sergey Lavrov on the margins of a multilateral meeting, telling him that Moscow must end its war against Ukraine immediately.

In what is believed to be the highest-level face-to-face conversation between the United States and Russia since the invasion started more than a year ago, Blinken said he urged Moscow to "engage in meaningful diplomacy that can produce a just and durable peace."

U.S. Secretary of State Antony Blinken (L) and Russian Foreign Minister Sergey Lavrov. (Kyodo) 

Their unscheduled encounter, which lasted less than 10 minutes according to a U.S. official, came as Washington's relations with Moscow, as well as with Beijing, grow tenser over the war and a number of other issues.

At a press conference following the Group of 20 foreign ministers' meeting in New Delhi, Blinken regretted that China and Russia had refused to endorse a joint statement condemning the war and reaffirming the need to uphold international law. The two countries were the only holdouts.

When asked about signs seen by the United States and NATO that China plans to provide lethal military aid to Russia, Blinken said that many countries expressed concern over the matter during the G-20 meeting.

He suggested the United States together with its partners would consider imposing sanctions against China should it decide to supply lethal weapons to Russia for its war effort.

During the conversation with Lavrov, the top U.S. diplomat said he also urged Moscow "to reverse its irresponsible decision" and rejoin the New START treaty. Moscow recently withdrew from the pact, which was the only remaining nuclear arms control treaty with Washington.

"Mutual compliance is in the interest of both our countries. It's also what people around the world expect from us as nuclear powers," Blinken said.

"I told the foreign minister that no matter what else is happening in the world or in our relationship, the United States will always be ready to engage and act on strategic arms control, just as the United States and the Soviet Union did even at the height of the Cold War," he added.

Russian media reported that Blinken had requested contact with Lavrov. Tass quoted Russian Foreign Ministry spokeswoman Maria Zakharova as telling the news agency, "But there were no negotiations, no meeting or so on."

U.S. State Department spokesman Ned Price said in a press briefing in Washington on Thursday that the United States is not expecting any more official dialogue at a senior level in the near future.

"Of course, we're always going to remain open to dialogue," Price said, given that the bilateral relationship has grown particularly tense. "But there was no agreement or consensus coming out of that brief encounter for any sort of follow-up discussion."