VIENNA (REUTERS) - The Russian military appears to have been involved with the armed groups who set up road blocks and prevented an unarmed OSCE observer mission from entering Ukraine's Crimea region last week, the United States said on Wednesday.

The US diplomatic mission to the Organisation for Security and Cooperation in Europe (OCSE) quoted a report by the OSCE observer team which said the gunmen appeared to have Russian military equipment and vehicles.

Pro-Russian forces have taken over military installations across Crimea, home to the Russian Black Sea Fleet. When the OSCE military obsever team tried to get into Crimea last Saturday, warning shots were fired and it was turned back.

"The passage into Crimea of the multinational delegation sent to Ukraine... was blocked five times in multiple locations by heavily armed guards lacking clear national identification," a US statement said.

It quoted the team's report as saying that its observations "produced significant evidence of equipment consistent with the presence of Russian Federation military personnel (in the vicinity of) the various roadblocks encountered during the period of the observation".

It also said trucks had number plates associated with the Black Sea Fleet, according to the US statement.

The US ambassador to the OSCE Daniel Baer said in the statement: "This report adds to our deep concerns and clearly suggests direct involvement by the Russian federation and its agents in preventing impartial, unarmed observers from doing the work they are supposed to do."

Moscow, which denies its troops have a role in the takeover of Crimea, says people there - a small majority of whom are ethnic Russians - should have the right to secede. A referendum on joining Russia is to be held on Sunday.