The Russian defense ministry said on Saturday its air defense forces shot down four U.S.-produced long-range missiles over the Crimea peninsula, weapons known as Army Tactical Missile Systems (ATACMS) that Washington has shipped to Ukraine in recent weeks.

The ministry said later that Russian aircraft and air defense systems had downed a total of 15 ATACMS in the past week.

On Tuesday, Russian officials said Ukraine had attacked Crimea with ATACMS in an attempt to pierce Russian air defenses of the annexed peninsula but that six had been shot down.

A U.S. official said in Washington last month that the United States secretly shipped long-range missiles to Ukraine in recent weeks.

The ATACMS missiles, with a range up to 190 miles were used for the first time in the early hours of April 17, launched against a Russian airfield in Crimea that was about 103 miles from the Ukrainian front lines, the official said.

The Pentagon initially opposed the long-range missile deployment, concerned that taking the missiles from the American stockpile would hurt U.S. military readiness.

There were also concerns that Ukraine would use them to attack targets deep inside Russia, a step which could lead to an escalation of the war towards a direct confrontation between Russia and the United States.

Separately on Saturday, the Russian defense ministry said that in the last week its forces had destroyed a military train carrying equipment and arms produced in the West and supplied to Ukraine by NATO.

The scale of the damage, exact date and location were not disclosed.

Reuters is not immediately able to corroborate battlefield accounts from either side.

On Thursday, British Foreign Secretary David Cameron promised $3.7 billion of annual military aid for Ukraine for "as long as it takes," adding that London had no objection to its weapons being used inside Russia, drawing a strong rebuke from Moscow.