The U.S. Defense Department acknowledged it’s upgrading nuclear weapons stored abroad, but — in keeping with standard practice — didn’t specify where.

“We are proceeding with full-scope nuclear weapon life extension programs to ensure reliability and enhance surety, safety, security, and use control, including for those weapons that are forward deployed,” Navy Capt. Jeff Davis, a Pentagon spokesman, said on Wednesday during a briefing with reporters.

Davis wouldn’t confirm whether the work is taking place Incirlik Air Base in Turkey near the Syrian border. “We neither confirm nor deny their presence at any specific location,” he said.

His comments came after Maria Zakharova, the Russian Foreign Ministry spokeswoman, told German’s ZDF television that the U.S. was modernizing B61 nuclear bombs stored “not just in Germany, but also in Belgium, the Netherlands, Italy, and Turkey.”

Her comments follow a recent report from a U.S. watchdog group that concluded the Air Force was upgrading two of its European bases — Incirlik Air Base in Turkey and Aviano Air Base in Italy — to better secure nuclear weapons stored there.

The report, which cited commercial satellite imagery, came from the Federation of American Scientists, a nonprofit based in Washington, D.C., which opposes the proliferation of nuclear weapons.

The construction work — which includes new security fencing, lighting and sensors — raises doubts about the security of the nuclear weapons stored in the region, according to Hans Kristensen, director of the Nuclear Information Project at FAS.

“The upgrades indirectly acknowledge that security at U.S. nuclear weapons storage sites in Europe has been inadequate for more than two decades,” he wrote. “And the decision to upgrade nuclear security perimeters at the two U.S. bases strongly implies that security at the other four European host bases must now be characterized as inadequate.”

Incirlik Air Base is the largest nuclear weapons storage site in Europe with 25 underground vaults, each of which can hold up to four bombs for a maximum total base capacity of 100 bombs, according to FAS. There are an estimated 50 B61 thermonuclear bombs stored there — a quarter of the U.S. stockpile of the weapon, which can be carried by F-16s and other aircraft.

The Defense Department earlier this month announced it was offering voluntary evacuations to almost 1,000 mostly Air Force family members from Incirlik “out of an abundance of caution” as unrest spread in the region. The U.S. recently began using the airbase — located less than 100 miles from the Syrian border — to launch airstrikes against Islamic State militants in Syria.

Russia, which in recent days has accelerated its military buildup in Syria to prop up the regime of President Bashar al-Assad, threatened to respond to the U.S. nuclear work in the region, possibly by placing Iskander ballistic missiles in the Russian enclave of Kaliningrad on the Baltic Sea.

“This may upset the strategic balance in Europe,” Dmitry Peskov, spokesman for Russian President Vladimir Putin, said of the American actions. “Hence, Russia will naturally have to take counter-moves, counter-measures for restoring this strategic balance and parity.”

-Richard Sisk contributed to this report.