By Alessandra Prentice and Sergiy Karazy

KIEV, March 2 (Reuters) - Ukrainian Defence Minister Stepan Poltorak on Friday said the decision by the United States to approve the supply of anti-tank missiles opened the door for closer military cooperation in the face of Russian aggression.

On Thursday, the Pentagon announced that the sale of Javelin missiles and launch units to Ukraine had been approved - a move the United States has been mulling since early 2015 to help Ukraine in its standoff with Russia-backed separatists.

"Unfortunately the issue of giving lethal weapons was under consideration for a long time," Poltorak said in an interview with Reuters. "But we received it (the decision) when we received it."

"We have all possibilities to cooperate with the United States and to increase our cooperation including in the defence sector," he said. He didn't give further details.

The United States has been one of Kiev's staunchest supporters since Moscow's annexation of Crimea in 2014 and the subsequent outbreak of fighting in Ukraine's eastern Donbass region that has killed more than 10,000 people.

But the decision to provide lethal aid is sensitive. The Kremlin has repeatedly said supplying weapons to Ukraine would further destabilise the situation by encouraging Kiev to use force.

Russia denies accusations from Kiev and NATO that it supports the rebels with troops and weapons. Clashes between government troops and the rebels continue despite a notional ceasefire and diplomatic peace efforts.

Speaking in a ministerial room decorated with photos of troops on the eastern front line, Poltorak said the weapons would help the Ukrainian military respond to attacks that he said Russia was responsible for coordinating.

"Its policy was aggressive and remains so. That's why the decision by the United States to supply lethal weapons is important for us," he said.

"Putin primarily uses the Donbass not only as a training ground to train servicemen for deployment in other hotspots, including Syria, and not only to test new weapons, equipment and other instruments. But he also uses it to prevent Ukraine from developing quickly," he said.

It is not clear when the first supplies of anti-tank missiles will be delivered to Ukraine, but Poltorak said preparations were already underway in terms of selecting personnel and safe storage for an expected delivery this year.

According to the Pentagon, Ukraine has asked to buy 210 Javelin missiles and 37 Javelin Command Launch Units. The prime contractor will be a joint venture of Raytheon Co (RTN.N) and Lockheed Martin Corp (LMT.N) .