The United States warned China against providing military or financial help to Moscow after its invasion of Ukraine, as sanctions on Russian political and business leaders mounted and civilians sought to flee intense fighting on the ground.
Further talks between Ukrainian and Russian negotiators to ease the crisis were expected on Tuesday after discussions on Monday via video ended with no new progress announced.
Two powerful explosions rocked the capital Kyiv before dawn, and air raid sirens were heard in different regions including Odessa, Chernihiv, Cherkasy and Smila.
Thousands have been killed in intense fighting and bombardments since Russian President Vladimir Putin ordered the invasion of Ukraine on February 24.
Russia calls its actions a "special military operation" to "denazify" the country and prevent genocide, a claim the United States and its allies reject as a pretext for an unjustified and illegal attack.
According to US officials, Russia has asked for military and economic support from Beijing, which signalled a willingness to provide aid.
Moscow denies that, saying it has sufficient resources to fulfil all of its aims. China's foreign ministry has labelled the reports on assistance as "disinformation".
"We have communicated very clearly to Beijing that we won't stand by," State Department spokesperson Ned Price told reporters after US national security adviser Jake Sullivan met with China's top diplomat Yang Jiechi in Rome. "We will not allow any country to compensate Russia for its losses."
The seven-hour meeting was "intense" and reflected "the gravity of the moment," according to a US official.
In Russia, a rare anti-war protest occurred in a studio during the main news programme on state TV's Channel One, which is the primary source of news for millions of Russians and closely follows the Kremlin line.
A woman held up a sign in English and Russian that said: "NO WAR. Stop the war. Don't believe propaganda. They are lying to you here." read more
Britain's defence ministry said Russia could be planning to use chemical or biological weapons in Ukraine in response to a staged fake attack on Russian troops, without citing evidence. US officials have made similar statements.
Russia has accused Ukraine of planning to use biological weapons. The United Nations on Friday said it had no evidence Kyiv had such a programme.
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Moscow on Monday allowed the first convoy to escape besieged Mariupol, home to the worst humanitarian crisis of the conflict.
"In the first two hours, 160 cars left," Andrei Rempel, a representative of the Mariupol city council told Reuters.
Local authorities say anywhere between 2,300 and 20,000 civilians have died so far in Russian shelling in the city, a toll that cannot be independently confirmed.
The United Nations says more than 2.8 million people have now left Ukraine since the start of the war.
"I am fleeing with my child because I want my child to stay alive," said a Ukrainian woman named Tanya who said she travelled from the town of Mykolaiv in southern Ukraine across the Danube river to Romania. "Because the people that are there now are Russians, Russian soldiers, and they kill children."
Russia says it does not target civilians.
EU member states agreed on Monday to a fourth package of sanctions against Russia, according to France. read more
Details were not officially disclosed, but diplomatic sources said they would include an import ban on Russian steel and iron, an export ban on luxury goods and a ban on investment in the energy sector. Chelsea soccer team owner Roman Abramovich and 14 others would be added to the EU blacklist, the sources said.
Japan on Tuesday announced an asset freeze for 17 Russian individuals, including 11 members of the Russian Duma, or parliament, five family members of banker Yuri Kovalchuk, as well as billionaire Viktor Vekselberg after similar US moves. read more
Western-led sanctions have cut Russia off from key parts of global financial markets and have frozen nearly half of the country's $640 billion gold and foreign exchange reserves, triggering the worst economic crisis since the 1991 fall of the Soviet Union.
Russia's finance ministry said it is preparing to service some of its foreign currency debt on Wednesday, but such payments will be made in roubles if sanctions prevent banks from honouring debts in the currency of issue.