Vladimir Putin attends a meeting with Xi Jinping in Beijing, China. File / Reuters

Xi's visit to Moscow is a diplomatic coup for Russian President Vladimir Putin, whose country has been placed under unprecedented international sanctions since he sent tens of thousands of troops into Ukraine on Feb. 24, 2022.

Beijing and Moscow struck a "no limits" partnership shortly before the invasion and U.S. and European leaders have said they are concerned Beijing may send arms to Russia.

China has denied any such plan, criticising Western weapon supplies to Ukraine, which will soon extend to fighter jets after Poland and Slovakia this week approved deliveries. The Kremlin said the jets would simply be destroyed.

A Russian official said Xi and Putin would discuss the conflict in Ukraine as well as "military-technical cooperation". The Kremlin said "important" bilateral documents would be signed, without elaborating.

Chinese foreign ministry spokesperson Wang Wenbin said Xi's visit to Russia - his first in nearly four years - would strengthen economic partnerships and promote "peace", although he made no explicit mention of the Ukraine war.

China is Russia's most important ally and has been buying Russian oil and other goods shunned by Western countries. It is also a big buyer of Ukrainian grain.

White House national security spokesman John Kirby said

the United States had deep concerns that China might try to position itself as a peacemaker in the war by promoting a ceasefire.

But any ceasefire at this time would not lead to a just and lasting peace between Ukraine and Russia, Kirby told a news briefing.

An international agreement to allow the safe export of grain from several Ukrainian ports on the Black Sea expires on Saturday. Russia has agreed to extend the deal for 60 days while Ukraine, along with the United Nations and Turkey which brokered the original deal, have called for a 120-day rollover.

Russia has not specifically said why it is insisting on the shorter period, although it has complained that its own food and fertilizer exports are being hindered by Western sanctions.

The United Nations Office in Geneva said on Friday that discussions were continuing on the renewal of the deal, needed to prevent global food shortages because Ukraine is a big exporter. A shorter extension would further restrict exports from Ukraine, which remain well below pre-invasion levels.

China, which has not condemned Russia's invasion, expressed concern about the war intensifying after a U.S. surveillance drone intercepted by Russian jets crashed into the Black Sea on Tuesday, in the first known direct U.S.-Russia confrontation.

Russian Defence Minister Sergei Shoigu presented awards to the pilots of the jets on Friday, Russian state-owned news agency RIA reported, a pointed retort to White House comments that the incident was reckless and perhaps showed incompetence.

Beijing has called for peace talks between Moscow and Kyiv, but Russia says Ukraine must accept the loss of four regions along with Crimea, which it forcibly annexed in 2014.

Ukraine says Russian troops must withdraw beyond its 1991 borders - the year the Soviet Union dissolved - and also that Moscow would use a truce to rebuild forces for a fresh assault.

Some media reports said Xi would hold a telephone call with Ukrainian President Volodymr Zelenskiy after his Russia visit but Beijing has not confirmed a call and a spokesperson for Zelenskiy told Ukrainian media it had not yet been agreed.

Pentagon spokesman Brigadier General Patrick Ryder told CNN it was important that China heard Ukraine's perspective to understand the impact of its support to Russia.