Russia has moved more troops closer to Ukraine's borders in recent days despite assurances it will not invade, US Defense Secretary Chuck Hagel said Wednesday.

Although Russia's defense minister Sergei Shoigu had told Hagel last week that Moscow would not send troops into eastern Ukraine, "the reality is that they continue to build up their forces," Hagel said.

"So they need to make sure they stay committed to what Minister Shoigu told me," the Pentagon chief said at a joint news conference after meeting his British counterpart, Philip Hammond.

And in another worrying sign, the British minister said it was unclear whether the Russian defense minister carries real influence over decision-making in Moscow, given President Vladimir Putin's dominant role.

"All the evidence suggests that the Russian agenda is being very much run by President Putin personally," Hammond said.

"And other Russian players, including Minister Shoigu, may express views, but it's a moot point, and we cannot know, we do not know to what extent all of those people are really inside the inner circle in which President Putin is planning this exercise," Hammond said.

There are now more than 20,000 Russian troops, including airborne units, fighter aircraft and armored vehicles, deployed near the Ukrainian border, providing ample firepower to seize the eastern region if Moscow chose to, according to US defense officials.

Hammond said diplomatic and economic sanctions against Russia over its intervention in Ukraine's Crimean peninsula were designed to isolate Moscow over its "aggression" while discouraging any further incursions.

"We've set out our positions partly as a response to what has already happened, but also with a focus on the practical need to deter any further aggression," he said.

Amid concerns among NATO states in Eastern Europe over Russia's actions in Ukraine, Hammond vowed that the alliance was fully committed to defend the sovereignty of its members.

"And I have no doubt -- I don't think any member of NATO has any doubt -- that all 28 members are prepared to come to the security interests, if that's what's required, to defend the integrity and sovereignty of those member countries," he said.

Asked about Ukraine's request to Washington for weapons and other lethal and non-lethal assistance, Hagel said the US administration had approved sending military rations to Kiev while other items were still under review.

"The president's national security team is reviewing all of the other requests for assistance, particularly the non-lethal assistance to Ukraine," Hagel said.

The crisis in Ukraine had given fresh meaning to the importance of the NATO alliance, the US and British defense chiefs said, and underscored the need to keep up defense spending despite budget pressures.

"We both recognize that defense budgets come down as wars come to an end," Hagel said.

"But we also recognize that there is a certain level of investment that any country must continue to make to protect its citizens and honor its security commitments."