The United States has formally approved the sale of Javelin anti-tank missiles to Ukraine, the State Department said Thursday, confirming a move expected to anger Kiev's foe Russia.

The Defense Security Cooperation Agency said Ukraine has asked President Donald Trump's U.S. administration for permission to buy 210 missiles and 37 launchers at a cost of around $47 million.

U.S. government employees and contractors will help transport the weapons and train Ukraine's forces, who are fighting a Russian-backed insurgency in the east of the country, to use them.

"The Javelin system will help Ukraine build its long-term defense capacity to defend its sovereignty and territorial integrity in order to meet its national defense requirements," the statement said.

Since Moscow's annexation of Crimea in 2014, Ukraine and Russia have been at loggerheads over a war in eastern Ukraine between pro-Russian separatists and Ukrainian government forces that has killed more than 10,000 people in three years.

Ukrainian President Petro Poroshenko said in December the weapons would be used to protect Ukrainian soldiers and civilians, but Russia said the U.S. decision would encourage those who support the conflict in Ukraine to use force.

The Pentagon said in a statement on Thursday the proposed sale would not alter the military balance in the region.

If no-one in the U.S. Congress objects to the sale, contractors Raytheon and Lockheed Martin will be able to begin supplying the weapons within two months.

The move comes the same day that Russian President Vladimir Putin announced his country has developed new nuclear weapons he claims can't be intercepted by an enemy.