Russia has warned that it will fire on western armaments shipments to Kyiv, raising the risk of a direct military confrontation between Moscow and Nato during the war in Ukraine.

Deputy foreign minister Sergei Ryabkov said on Saturday that “pumping up [Ukraine] with weapons from a whole range of countries” was “not just a dangerous move — it’s something that turns these convoys into legitimate military targets”, according to the Interfax news agency.

US president Joe Biden has rejected calls from Kyiv to establish a no-fly zone over Ukraine and said Nato will not be drawn into an all-out conflict with Russia or send troops to defend the country.

However, Ukraine’s western backers have pledged significant military aid to the country. The US has promised $6.5bn in defence spending, the UK has sent 3,615 NLAW anti-tank missiles and Javelin anti-tank weapons while Nordic states have sent more than 10,000 anti-tank weapons.

Russian forces continued their assault on Ukraine on Saturday with fierce fighting on the outskirts of Kyiv and reports of a strike on an airfield at Vasylkiv, south of the capital. Ukraine police said that an ammunition depot was damaged, shells detonated and a fire broke out.

“The airport is completely destroyed. An arsenal of weapons exploded,” said Natalia Balasynovych, Vasylkiv’s mayor.

A food distribution centre to the west of Kyiv was also hit by shelling.

Russia pressed on with its aerial bombardment of the besieged city of Mariupol and struck a mosque there, according to Ukrainian authorities. There was no immediate confirmation of casualties.

There were street protests in Melitopol where Vladimir Putin’s forces have detained Ivan Fedorov, the city’s mayor.

Ukraine’s president Volodymyr Zelensky called upon world leaders to pressure Moscow to release him. “We appeal to all world leaders who speak to Moscow. France, Germany, Israel and others,” said Zelensky.

The Ukrainian president said Russian losses were “simply astounding”, claiming the destruction of more than 360 tanks, 1,205 armoured vehicles, “almost 60 planes [and] more than 80 helicopters”. His claims could not be independently verified.

German Chancellor Olaf Scholz and French President Emmanuel Macron held a joint phone call with Putin on Saturday. In the 75-minute conversation they “urged an immediate ceasefire in Ukraine and a diplomatic solution to the conflict”, the chancellery said in a statement. Scholz had spoken to Zelensky earlier in the day.

With the war in its third week, the west has stepped up sanctions on Russia. On Friday G7 nations said they would end normal trade relations with Moscow.

The joint step includes revoking Russia’s “most-favoured nation” status which allows the country to trade goods on preferential terms with many western countries under rules set by the World Trade Organization. The move will lead to higher tariffs on many Russian exports.

The G7 also agreed on other measures, including stopping Russia from obtaining any financing from international institutions such as the IMF and the World Bank.

In the latest step against Russia’s oligarchs, Italian authorities seized a superyacht valued at $530mn owned by billionaire Andrey Melnichenko in the port of Trieste.

Ryabkov accused the US of further inflaming tensions by leading the western sanctions effort. “It’s the US that has ramped up tensions to the maximum in international affairs. And that’s before we get to what they’re doing in practice by supporting the criminal Kyiv regime,” Ryabkov said.

He added that Russia and the US were in “constant contact” and Moscow was still open to a strategic dialogue with Washington on arms control.

Putin is demanding that Ukraine recognises its 2014 annexation of the Crimean peninsula and the independence of two separatist statelets in the Donbas region, commits to demilitarisation and declares neutrality in its constitution.

Ukraine has said it is open to making some concessions but has described Moscow’s stance as a demand for “capitulation”. Talks in Belarus and Turkey this week failed to reach a breakthrough or even establish a ceasefire, with foreign minister Sergei Lavrov denying that Russia had attacked Ukraine.

Meanwhile, Biden said on Friday that Russia would pay a “severe price” if it used chemical weapons during its war on Ukraine, although he declined to specify the nature of any US response.

US officials have said in recent days that Russia might resort to the use of chemical weapons after first setting up a pretext by deploying them in a “false flag” operation and blaming it on Ukraine.