Russia wants to secure a land corridor in eastern Ukraine that could include Mariupol but pro-Moscow rebels may wait until the spring to attack the strategic port city, the US spy chief said Thursday.

Russian President Vladimir Putin's aim was not to seize all of Ukraine but to carve out territory linking up with the Crimean peninsula, which was annexed by Moscow last year after the fall of a pro-Moscow leader in Kiev, James Clapper told lawmakers.

"It is not our assessment that he is bent on capturing or conquering all of Ukraine," Clapper told the Senate Armed Services Committee.

"He wants a whole entity composed of the two oblasts (regions) in eastern Ukraine which would include a land bridge to Crimea and perhaps a port in specifically Mariupol.

"We do not believe that an attack on Mariupol is imminent. I believe they will wait until the spring before they attack."

Since a ceasefire brokered by European leaders went into effect on February 15, the rebels have captured the key rail hub of Debaltseve and were reported to be massing forces around Mariupol, a key port on the Sea of Azov.

Clapper said he favored sending weapons to Kiev to help the Ukrainian army in its fight against the separatists but made clear that was his "personal view" and not necessarily the position of US intelligence agencies.

President Barack Obama is weighing the idea of arming Ukraine but has yet to announce a final decision. European allies have warned such a move would escalate the conflict without preventing Russia from achieving its aims.

US spy agencies believed that arming Kiev "would evoke a negative reaction from Putin and the Russians" and could prompt Moscow to send more advanced weapons to the rebels, Clapper said.

At the same hearing, the head of military intelligence, Lieutenant General Vincent Stewart, declined to offer his personal opinion.

But he said the Defense Intelligence Agency concluded lethal aid could not be delivered quickly enough to counter the pro-Russian forces and would not "change the military balance of power on the ground."

Russia -- which denies it is backing the rebels -- placed a top priority on preventing Ukraine from joining the European Union or NATO and "they will up the ante if we do any lethal aid or take any actions to bolster the equation," Stewart said.

The Obama administration so far has stopped short of sending weapons to Ukraine, but has provided radios, body armor, medical equipment and counter-mortar radar to the government army.