President Barack Obama's aides and top commanders are seriously considering providing arms and more military equipment to Ukraine as its army struggles against pro-Russian separatists, officials said Monday.

The Obama administration had previously ruled out sending weapons to Ukraine's government but the failure of economic sanctions to persuade Russia to halt military assistance for the separatists has prompted a second look at the option, officials told AFP.

Some senior figures in the administration now backed the move despite earlier concerns about triggering a dangerous escalation with Russia, officials said.

Washington so far has provided non-lethal assistance to Ukraine, including flak jackets, medical supplies, radios and night-vision goggles.

"What's being discussed is perhaps we should begin providing defensive weapons, defensive equipment, to Ukraine," a senior defense official said.

The view on what to provide Ukraine "has matured" given Russia's backing of the separatists and recent violations of a ceasefire agreement, a second official said.

The discussion inside the administration comes as Russia has ramped up deliveries of tanks and other military hardware to separatists in eastern Ukraine over the past month, officials said.

The New York Times first reported the policy shift, which coincided with an appeal by a group of former senior civilian and US military leaders urging Washington to supply arms to the Ukrainian government.

Authors of the report included officials with close ties to the White House, including the former number-three-ranking civilian at the Pentagon, Michele Flournoy, and the former US ambassador to NATO, Ivo Daalder.

"The West needs to bolster deterrence in Ukraine by raising the risks and costs to Russia of any renewed major offensive," said the report published by the Atlantic Council, the Brookings Institution and the Chicago Council on Global Affairs.

"That requires providing direct military assistance -- in far larger amounts than provided to date and including lethal defensive arms -- so that Ukraine is better able to defend itself," said the report, which was signed by former NATO commander Admiral James Stavridis and the former deputy commander for US forces in Europe, General Charles Wald.

The report called for $3 billion in military assistance over the next three years, including providing light-armor missiles and armored Humvee vehicles.

- Countering jamming, artillery fire -

The head of US Army forces in Europe, Lieutenant General Ben Hodges, said last week that the Ukrainian forces faced difficulties on the battlefield because they lacked radar and drones to track incoming artillery fire and equipment to safeguard their communications against jamming.

General Philip Breedlove, NATO's supreme allied commander, was among those advocating arms deliveries for Kiev, according to The New York Times.

His spokesman declined to say whether the general favored a change in approach.

"General Breedlove has repeatedly stated that he supports the pursuit of a diplomatic solution as well as considering practical means of support to the government of Ukraine in its struggle against Russian-backed separatists," Captain Gregory Hicks said.

Breedlove backed efforts by US and Ukrainian officials "to improve the capability and capacity of Ukraine's armed forces," he said.

Secretary of State John Kerry was due in Kiev this week for talks and the State Department said Washington is keeping all options open.

His spokeswoman, Jen Psaki, said: "We are constantly assessing our policies on Ukraine to ensure they are responsive, appropriate, and calibrated to achieve our objectives."

Washington was "particularly concerned about recent escalating separatist violence and separatist attempts to expand the territory they control," Psaki told reporters.

But she said no decisions had been taken on the issue.

Psaki refused to go into "internal policy discussions" but stressed Washington was taking into account "events on the ground."

"I don't think anybody wants to get into a proxy war with Russia. And that is not the objective. Our objective here is to change the behavior of Russia," she added.