KYIV: President Volodymyr Zelenskiy said on Monday that vital U.S. weapons were starting to arrive in Ukraine in small amounts and that the process needed to move faster as advancing Russian forces were trying to take advantage.

Zelenskiy told a joint press conference in Kyiv alongside visiting NATO chief Jens Stoltenberg that the situation on the battlefield directly depended on the speed of ammunition supplies to Ukraine.

"Timely support for our army. Today I don't see anything positive on this point yet. There are supplies, they have slightly begun, this process needs to be sped up," he said.

The United States passed a $61 billion aid package last week, ending months of congressional deadlock and raising hopes in Kyiv that its critically low stocks of artillery shells would soon be replenished.

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Ukraine's top commander said on Sunday his troops had pulled back from three villages in the east where Russian forces are on the offensive and gradually taking ground.

"The Russian army is now trying to take advantage of a situation when we are waiting for supplies from our partners... and that is exactly why the speed of deliveries means stabilising the front," Zelenskiy said.

"Russia is preparing for offensive actions," he added.

The Ukrainian leader also singled out the need for supplies of sophisticated Patriot air defence launch systems that Kyiv wants to obtain from its allies to counter Russia's long-range missile, and drone strikes.

He said Kyiv had made some progress in obtaining supplies of missiles for the Patriot system, but that it was still working on the question of obtaining new launchers that fire the missiles.