Ukraine border guards say an observation plane has come under fire while on patrol in the Crimean border region.

There are no reported injuries, according to Reuters, but the latest development comes as Russian President Vladimir Putin is said to be reinforcing the military presence in the disputed region.

A convoy of 60 military trucks bearing Russian registration plates and carrying hundreds of soldiers was seen travelling from eastern Ukraine into Crimea on Saturday.

The convoy is said to have travelled west of Feodosia to a military airfield at Gvardeiskoe, north of Simferopol, according to the Associated Press.

Earlier today pro-Russia gunmen fired warning shots into the air to prevent international military observers from crossing the Crimean border and entering Ukraine, for the third day running.

The team of 43 unarmed inspectors from 23 countries, who had been invited from the Organisation for Security and Cooperation in Europe (OSCE) to monitor the situation in the region, were forced to abandon their mission and return to the Ukrainian town of Kherson, halfway between Odessa and the Crimean peninsula.

Pro-Russian forces took control of key installations on the peninsula on 28 February in a largely bloodless operation which seems to have had the support of many of Crimea's majority ethnic Russians.

The OSCE mission had aimed to defuse a tense military standoff. Forces loyal to Moscow bloodlessly seized Crimea after Russian ally Viktor Yanukovich was ousted as Ukrainian president last month, and are surrounding military compounds of the Ukrainian army and navy.