Pakistan has doubled its nuclear weapons stockpile over the past several years, increasing its arsenal to more than 100 deployed weapons, according to a published report.

As a result, Pakistan has now edged ahead of India, its nuclear-armed rival, The Washington Post reported late Sunday, citing non-government analysts. India is estimated to have 60 to 100 weapons.

Only four years ago the Pakistani nuclear arsenal was estimated at 30 to 60 weapons.

"They have been expanding pretty rapidly," the report quoted David Albright, president of the Institute for Science and International Security, as saying.

Based on recently accelerated production of plutonium and highly enriched uranium, Islamabad may now have an arsenal of up to 110 weapons, Albright said.

"Pakistan lives in a tough neighborhood and will never be oblivious to its security needs," Brig. Gen. Nazir Butt, a defense official at the Pakistani Embassy in Washington, said. "As a nuclear power, we are very confident of our deterrent capabilities."

The report came as hard-line Islamic leaders rallied at least 15,000 people against an American official arrested in the shooting deaths of two Pakistanis and warned the government not to cave in to U.S. pressure to release the man.

The protest in the eastern city of Lahore, where the shootings took place, came as the U.S. Embassy once again insisted that the American has diplomatic immunity and was being detained illegally by Pakistan. But Pakistan has refused to budge, saying the matter must be decided by the courts.

The spat has revealed the fragility of a relationship Washington believes is crucial for success in Afghanistan and against al-Qaida. Large protests by hard-line Islamic groups, which have significant influence in Pakistan, could make it even more difficult for the government to free the American.

"We warn the government and administration that ... if they help the arrested American illegally, then this crowd will surround the U.S. Embassy and presidential palace in Islamabad," Hafiz Hussain Ahmed, a senior official in the Jamiat Ulema Islam party, said during Sunday's rally.

The U.S. has said the American, who has not been named, acted in self-defense when he shot two armed men who approached his car in Lahore on Thursday.

But many questions have been left unanswered, including exactly what the American did at the U.S. Embassy and why he was carrying a gun. The lack of clarity has fueled media speculation he may have been a CIA agent or security contractor, as well as questions over whether he qualified for diplomatic immunity.

The Associated Press contributed to this report.