ISLAMABAD — Al-Qaida has claimed responsibility for the kidnapping of an American man in Pakistan and demanded the release of prisoners and an end to air strikes in Muslim countries in exchange for his freedom, according to an Internet statement.

Al-Qaida leader Ayman al-Zawahri also said in an audio recording issued on Islamist websites late on Thursday that a senior al-Qaida leader based in Pakistan known as Attiyatullah had been killed in a U.S. air strike in August.

"Just as the Americans detain all whom they suspect of links to al-Qaida and the Taliban, even remotely, we detained this man who has had an active part in American aid to Pakistan since the seventies," SITE quoted Zawahri as saying in the recording.

The U.S. State Department is aware of the statement and continues to work with Pakistani authorities leading the investigation, a spokeswoman said.

Assailants kidnapped Warren Weinstein, an American development expert, in the Pakistani city of Lahore in August.

Weinstein, about 70 years old, had been working on a project in Pakistan's northwestern tribal areas where Pakistani troops have been battling Islamist insurgents for years.

"We remain concerned for Mr. Weinstein's safety and well-being," said Joanne Moore, spokeswoman for the State Department. The government had been in contact with Weinstein's family in the United States, she said.

"U.S. officials, including the FBI, are assisting in the Pakistani-led investigation," she said, declining to give additional information on the case due to privacy considerations.

"The United States condemns kidnappings of any kind and we call for the immediate release of the individual and the prosecution of those responsible," Moore said.

Zawahri said the group's demands for Weinstein's release included the release of all those held by the United States at the Guantanamo detention center and all others imprisoned for ties to al Qaeda or the Taliban.

He also demanded an end to air strikes by the United States and its allies against militants in Pakistan, Afghanistan, Yemen and Somalia and Gaza.

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Zawahri also demanded the release of high-profile militants including Ramzi Yousef, imprisoned in the United States for the 1993 bombing of the World Trade Center, and Sheikh Omar Abdel Rahman, serving a life sentence for plotting to attack the U.N. headquarters and other New York City landmarks.

"Your problem is not with us but with (President Barack) Obama. We have raised fair demands. ... So continue to pressure Obama, if you want your relative to be handed back," Zawahri said, addressing Weinstein's family.

Zawahri said that Attiyatullah, a Libyan militant whose real name was Jamal Ibrahim Ashtiwi al-Misrati, escaped a first air strike but was killed along with his son Issam in a second bombing on August 23.

"He was martyred, may God have mercy on him ... by bombing by a crusader spy plane," Zawahri said.

Zawahri was named by the Islamist group to succeed Osama bin Laden, who was killed in an operation by U.S. forces in Pakistan in May after a decade-long worldwide hunt.

Al-Qaida has tried to wage war on Arab rulers over the past decade through creating cells that used suicide attacks on foreigners and government installations and officials.

But the Arab Spring popular uprisings have left al-Qaida on the sidelines, as uprisings brought down veteran heads of state in Egypt, Tunisia, Libya and Yemen.