WASHINGTON (AP) — The Defense Department said Wednesday it is canceling plans to buy additional cargo helicopters from the Russian arms export agency that has supplied Syrian President Bashar Assad's military forces with arms and ammunition.

The additional 15 Russian-built Mi-17 helicopters were to be purchased next year at a cost of $345 million and then delivered to Afghanistan's national security forces.

Bipartisan opposition to the Mi-17 acquisition grew as the violence in Syria escalated and U.S. relations with Russia deteriorated. A growing number of lawmakers from both political parties objected to acquiring military gear from Rosoboronexport, which has provided Assad's regime with weapons used against Syrian civilians.

"I applaud the Defense Department's decision to cancel its plan to buy 15 additional Mi-17 helicopters from Rosoboronexport," Sen. John Cornyn, R-Texas, said in an emailed statement. "Doing business with the supplier of these helicopters has been a morally bankrupt policy, and as a nation, we should no longer be subsidizing Assad's war crimes in Syria."

Cornyn, the Senate's No. 2 GOP leader, said he was informed of the decision last week by Deputy Defense Secretary Ashton Carter.

Pentagon spokeswoman Maureen Schumann said the department re-evaluated the requirements for Mi-17s in consultation with Congress. "We currently do not have plans to purchase additional Mi-17s from Rosoboronexport beyond those" already under contract.

Top U.S. military officials have maintained the Russian-made helicopters are ideally suited for the Afghans, who are rebuilding their air force and need a reliable and easy-to-operate helicopter for transporting troops throughout the country's harsh environment. Overall, the Defense Department has paid more than $1 billion since 2011 for 63 Mi-17s that have been delivered to Afghanistan or are on order.

Russian Embassy spokesman Yevgeniy Khorishko played down the Pentagon's decision. He described it as a single action that doesn't affect the broader military and technical cooperation between the two countries.

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Rosoboronexport announced Monday that 12 of the Mi-17s had been delivered to Afghanistan in the month of October. The shipments, the export agency said, reflected the joint effort between Russia and the U.S. to combat international terrorism.

Frank Kendall, the undersecretary of defense for acquisition, told Cornyn in a letter last year that the Defense Department "condemns the actions of Rosoboronexport in supplying arms and ammunition to the regime of Bashar al-Assad in Syria, whose forces have used these weapons to murder Syrian civilians."

Rosoboronexport's director general, Anatoly Isaykin, said in February that since there are no sanctions against shipping weapons to Syria, Russia was still fulfilling its contract obligations. "These aren't offensive weapons," he said. "We are mostly shipping air defense systems and repair equipment for various branches of the military."

The urgent need to supply the Afghans with Mi-17s had trumped congressional calls to terminate the contract with Rosoboronexport.

Cornyn and other members of Congress also argued the Defense Department should have more seriously considered acquiring an American-made helicopter for the Afghans. The U.S. Army's Chinook, manufactured by defense giant Boeing in Pennsylvania, and a transport helicopter made by Sikorsky in Connecticut, were among the possible options.

Rep. Rosa DeLauro, D-Conn., called the move a welcome, if long overdue, first step and she urged the Pentagon to spend U.S. tax dollars on American-made systems. "I am proud Congress united in a bipartisan manner to deliver that message, and that the Pentagon has finally heard us," she said.

Carter told House lawmakers in September that multiple reviews and assessments were conducted of more than two dozen helicopters that were either available or in development. Carter said the Afghans are very familiar with the Mi-17 and none of the other aircraft examined met the requirements.

Despite the Pentagon's certainty the Mi-17 was the right choice for the Afghans, Capitol Hill refused to let up its campaign to end the business relationship with the Russians.

In an August letter to Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, 13 Republican and Democratic senators questioned whether Afghanistan could ever be fully independent of Russia if the country continues to operate Russian aircraft for decades to come. They also expressed concern the arrangement could put the United States at a disadvantage on matters of strategic importance.

They questioned whether the "overreliance on Russia fostered by this Mi-17 program put the U.S. at risk of Russian coercion or blackmail on other security issues," including the crisis in Syria, Iran's drive to obtain nuclear weapons, and U.S. missile defense.

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