The United States raised fears Wednesday that civilians are being targeted by Russia in Ukraine, warning that Moscow is moving cluster munitions and other lethal weaponry into the country in a potentially dangerous new phase of the conflict.

One week after Russian President Vladimir Putin invaded his eastern European neighbor, top US diplomat Antony Blinken said the human costs are already "staggering."

"Hundreds if not thousands of civilians have been killed or wounded," Blinken, who will travel to eastern Europe next week to shore up US support for Ukraine, told a press briefing in Washington.

Russia has launched attacks on buildings and cities that "aren't military targets," he said, adding: "The humanitarian consequences will only grow in the days ahead."

The Secretary of State spoke after a warning earlier Wednesday by US ambassador to the UN Linda Thomas-Greenfield that Russia was moving "exceptionally lethal weaponry" into Ukraine.

That includes cluster munitions and vacuum bombs, which she told the General Assembly are banned under international law and have "no place on the battlefield."

A senior Pentagon official also warned that "we would expect to see artillery strikes increase" as Russian forces try to surround Ukrainian cities.

"The worry is that as they become more aggressive, they will become less precise and less discriminant," the official, who spoke on condition of anonymity, said.

Putin invaded Ukraine on February 24, igniting a global outcry and fierce resistance by Ukrainians.

A woman is comforted by a friend after arriving on a train from Ukraine’s border at Berlin’s main train station on March 2, 2022
A woman is comforted by a friend after arriving on a train from Ukraine’s border at Berlin’s main train station on March 2, 2022 AFP / Tobias SCHWARZ

Hundreds of civilians have been killed and hundreds of thousands of Ukrainians have fled since the invasion began, while the West has imposed sanctions to cripple Russia's economy.

Putin has also ordered his nuclear forces be mobilized in a move that sparked immediate condemnation.

Blinken slammed the Russian leader's nuclear rhetoric as "the height of irresponsibility" Wednesday, but said Washington was ready to support any diplomatic efforts to reach a ceasefire and the withdrawal of Moscow's troops.

Russia has mooted the possibility of ceasefire talks, and a Ukrainian delegation was on the way to negotiations on the Belarus-Poland border on Wednesday.

Pentagon spokesman John Kirby said meanwhile that the United States had postponed a planned test launch of a ballistic missile so as not to exacerbate tensions.

"We recognize at this moment of tension how critical it is that both the United States and Russia bear in mind the risk of miscalculation and take steps to reduce those risks," he said.

The State Department announced that Blinken will travel to Belgium, Poland, the Baltic states and Moldova from March 3-8 to reaffirm Washington's support for Ukraine.

The stop in Chisinau comes as Western leaders have raised concerns that Moscow's offensive against Kyiv could spread to Moldova, another former Soviet republic.

Moldova, whose president Maia Sandu was elected on a Western-facing platform, is home to the pro-Russian separatist region of Transdniestr.

Poland, meanwhile, is at the forefront of the effort to host refugees fleeing Ukraine. The UN says it has taken in more than 450,000 Ukrainians, or 51.9 percent of the refugees so far.