The United States said Thursday it was confident of the safety of Pakistan's nuclear arsenal after heavily armed militants stormed an air force base in clashes that left 10 people dead.

State Department spokeswoman Victoria Nuland offered condolences over the attack claimed by the Taliban. She said the United States had no reason to doubt Pakistan's account that the Minhas base was free of nuclear weapons.

"We have confidence that the government of Pakistan is well aware of the range of potential threats to its nuclear arsenal and has secured its nuclear arsenal accordingly," Nuland told reporters.

"We do talk about these issues and support Pakistani efforts to keep them secure -- we have for quite a long, long time. And we don't have any reason to be concerned at this moment," she said.

The Pakistan Air Force said nine attackers dressed in military uniforms and armed with rocket-propelled grenades and suicide vests carried out an early morning raid at the base in Kamra, 60 kilometers (35 miles) northwest of Islamabad.

The assault came as the United States and Pakistan take steps to enhance cooperation, which has been severely strained in recent months over incidents including the US raid that killed Osama bin Laden.

"Pakistanis have suffered more than their share at the hands of terrorists inside Pakistan, which speaks to our efforts to address this threat together," Nuland said.

Pakistan is the Islamic world's only declared nuclear weapons state and the arsenal is considered the crown jewel by the powerful military.

Given Pakistan's sensitivities, the United States has been careful when discussing nuclear safety in the country but news reports have said that Washington has contingency plans in case of a major crisis.