NEW YORK (Reuters) - New York City police are on alert to possible revenge attacks following the U.S. killing of American-born militant Anwar al-Awlaki in Yemen, police commissioner Ray Kelly said on Friday.

Awlaki, identified by U.S. intelligence as "chief of external operations" for al Qaeda's Yemen branch and a Web-savvy propagandist for the Islamist cause, was killed in a CIA drone attack in a remote Yemeni town, U.S. officials said.

"We know al-Awlaki had followers in the United States including New York City, and for that reason we remain alert to the possibility that someone might want to avenge his death," Kelly said in a statement.

"He was a powerful recruiter of terrorists in the United States," he said.

Kelly also welcomed the reported death of a second English-speaking al Qaeda operative, Samir Khan, an American of Pakistani origin who U.S. and Yemeni officials said they believed was killed in the same drone attack.

"Khan had extensive contacts in New York City and published the English language Inspire Magazine, which instructed lone wolves on how to build bombs at home, and in the most recent issue identified Grand Central Station as a target," he said.

Al Qaeda militants are blamed for flying hijacked planes into the World Trade Center twin towers in Lower Manhattan on September 11, 2001, causing them to collapse and killing nearly 3,000 people.

New York police and intelligence services say they have disrupted at least 11 plots against the city since the 2001 attacks. Kelly told the CBS News program "60 Minutes," which aired on Sunday, that New York police can shoot down an airplane if needed.

The New York Police Department built up extensive counterterrorism and intelligence divisions after the attacks of September 11, 2001, saying it could not rely on the federal government alone for protection from an attack.

(Reporting by Michelle Nichols, editing by Vicki Allen)