* Photos contradict Russian assertions about withdrawal
* U.S. says pictures taken in last few days
* NATO spokesman says Russia still has 40,000 troops on Ukraine's borders (Adds NATO spokesman's comments)
BRUSSELS, May 13 (Reuters) - The U.S. government released new satellite pictures on Tuesday which it said showed Russian forces were still near the Ukrainian border in recent days, contradicting Russian assertions they had been withdrawn.
President Vladimir Putin announced last Wednesday that the troops had moved, but NATO and the United States both said they had seen no sign of a Russian withdrawal from the frontier.
NATO officials have previously estimated Russia has around 40,000 soldiers close to Ukraine's border, exacerbating the worst East-West crisis since the Cold War.
The commercial satellite photos, published by the U.S. State Department and on the U.S. Mission to NATO's Twitter account, showed what appeared to be about 20 Russian helicopters at Belgorod, near the Ukrainian border, on May 9.
Alongside it, the U.S. government published another image of the same area on March 26, also showing helicopters. An earlier photograph, from March 22, showed no helicopters in the area.
A second satellite photograph, taken in the Rostov area on May 12, within 75 km (50 miles) of the Ukraine border, showed what the U.S. government said was a group of Russian armoured vehicles. The vehicles were also there in an earlier picture of the same area, dated March 23.
Colonel Martin Downie, spokesman for Allied Command Operations at NATO, said recent comments by Russia's leadership on a troop withdrawal did not reflect realities on the ground.
"The reality is that Russia continues to have 40,000 high readiness troops massed on Ukraine's border and another 25,000 troops in Crimea. The units on the latest satellite pictures show mechanised infantry, armoured vehicles and combat helicopters," he said in a statement.
"These units are there to intimidate Ukraine's government and they could be used for an invasion of Ukraine, if ordered by Moscow. We will continue to keep a close eye on Russia's actions near our borders," he said.
When NATO published similar satellite photographs in April showing a Russian military buildup near the Ukraine border, a Russian official disputed their authenticity, saying the images were from August last year.
The allegation was denied by NATO officials, who said the photographs were recent.
Ukraine is not a member of NATO and NATO officials have said the alliance will not get involved there militarily. NATO countries have sent planes, ships and some soldiers to reinforce eastern European members of NATO concerned about their security following Russia's annexation of Crimea. (Reporting by Adrian Croft; Editing by Tom Heneghan and Eric Walsh)