Ashton Carter, the White House's choice for defence secretary, earlier said he was "inclined" to start supplying arms.

The US has so far only provided "non-lethal" assistance to Ukraine.

Meanwhile, Nato is set to unveil what the bloc's chief says are "the biggest reinforcement" of its "collective defence since the end of the Cold War".

At a meeting in Brussels, Nato defence ministers will seek to reassure the alliance's nations in Eastern Europe and to deter a potential Russian threat to the Baltic states or other bloc members should the crisis in Ukraine spin out of control, the BBC's Jonathan Marcus reports.

Our correspondent says a brigade-sized spearhead force is to be established - about 4,000-5,000-strong. Its lead units will be able to move east at two days' notice.

Small command centres are also to be set up in Estonia, Lithuania, Latvia, Poland Bulgaria and Romania.

Russia denies accusations by Ukraine and the West that it is arming the rebels in eastern Ukraine and sending its regular troops across the border.

'President interested'

Mr Kerry Mr Kerry will meet Ukrainian President Petro Poroshenko and Prime Minister Arseniy Yatseniuk in Ukraine's capital Kiev on Thursday morning. A news conference is expected later in the day.

The issue of weapons deliveries to Ukraine and other US assistance is expected to be one of the main items on the agenda.

Mr Kerry will then travel to Munich to take part in the annual security conference in the southern German city.

On Wednesday, Mr Carter, who previously served as deputy secretary of defence, appeared in front of the Senate Armed Service committee to be questioned before a wider Senate vote.

"I'm very much inclined in that direction, mister chairman, because I think we need to support the Ukrainians in defending themselves," Mr Carter said when asked by Senator John McCain if he supported delivering "defensive weapons" to Ukraine.

"The nature of those arms, I can't say right now," he added.

White House spokesman Josh Earnest later stressed that President Obama - not Mr Carter - would make the final decision on the issue.

But Mr Earnest added: "The president is certainly interested in the view, the opinion and the insight of his national security team, including what we hope will be his soon-to-be-confirmed new secretary of defence, and the president will certainly take that advice into account."

The White House has accused Russia of fighting a proxy war - but has reportedly expressed fears that sending in weapons could trigger a tense confrontation with Russia and escalate the conflict.

However, President Barack Obama is now said to be reconsidering his position, accusing Russia of escalating the conflict in Ukraine's eastern Donetsk and Luhansk regions.

Fighting has intensified in recent weeks - leaving a September ceasefire in tatters.

Earlier this week a group of former senior US officials and officers urged a major increase in military assistance, including providing light-armour missiles designed to take out tanks and armoured vehicles.

And on Tuesday, a group of US senators called on President Obama and Nato "to rapidly increase military assistance to Ukraine to defend its sovereign borders against escalating Russian aggression".

Fighting in eastern Ukraine began last April, when separatists seized government buildings after Russia annexed Ukraine's southern Crimea peninsula.

More than 5,000 people have been killed in the conflict.

(BBC)