KYIV — Ukrainian President Volodymyr Zelenskyy slammed the latest US claims that the Kremlin is planning an invasion as soon as early next week, saying that they are "causing panic and not helping".

"Currently, the best friend of our enemies is panic in our country. And all this information only causes panic and does not help us," Zelenskyy said during a trip to the provinces on Saturday.

Russian President Vladimir Putin and US President Joe Biden are to hold a high-stakes telephone call on Saturday as tensions over a possibly imminent invasion of Ukraine escalated sharply the day before.

Before talking to Biden, Putin is also to have a call with French President Emmanuel Macron. The two met in Moscow earlier in the week to try to resolve the crisis.

Russia has massed troops near the Ukraine border and sent troops to neighbouring Belarus for joint exercises. Still, the Kremlin insistently denies that it intends to launch an offensive against Kyiv.

Adding to the sense of crisis, the Pentagon ordered an additional 3,000 US troops to Poland to reassure allies.

Biden has said the US military would not enter a war in Ukraine, but he has promised severe economic sanctions against Moscow in concert with international allies.

According to a US official familiar with the findings, the US acquired intelligence that Russia is looking at Wednesday as a target date.

The official, who was not authorized to speak publicly and did so only on condition of anonymity, would not say how definitive the intelligence was, and the White House publicly underscored that the US does not know with certainty whether Putin is committed to invasion.

However, US officials said anew that Russia's buildup of offensive air, land and sea firepower near Ukraine had reached the point where it could invade on short notice.

The US State Department ordered the withdrawal of its non-essential staff "due to continued reports of a Russian military build-up on the border with Ukraine, an indicator of potential significant military action," the embassy in Kyiv posted on Twitter on Saturday.

Earlier, a similar order was issued, asking families of embassy staffers in Kyiv to leave. Non-essential personnel were given the option to remain prior to the latest decision. — Agencies