By Mark Hosenball and Warren Strobel

WASHINGTON (Reuters) - U.S. officials are debating whether to tighten controls on the border with Canada and make it easier to revoke the passports of suspected militants, steps that could gain traction following two attacks in Canada this week.

The officials cautioned on Thursday that the discussions are in preliminary stages and that no immediate action appeared likely by either U.S. President Barack Obama's administration or Congress.

While there was no specific evidence of a new threat in the United States, federal and state authorities were on a heightened state of alert following a gunman's attack in Ottawa on Wednesday and another by an assailant in Quebec on Monday.

One official familiar with the matter said a main topic of discussion has been whether some northern border posts which are unmanned - but guarded by electronic sensors and alarms - should now be staffed with live personnel.

The officials, speaking on condition of anonymity, said that proposals circulating within government agencies could make it easier for the government to revoke U.S. passports.

Authorities in the United States, Canada, Australia and western Europe say they are alarmed by thousands of citizens who have traveled to Syria to fight in the conflict there. They fear that some battle-hardened fighters could return to their home countries and attempt terror attacks.

Secretary of State John Kerry, whose department issues passports, has authority to revoke them. The government regards passports as a privilege, not a right. But one official said there are provisions for challenging such decisions.

The United States has used existing powers to cancel passports for counter-terrorism purposes, revoking that of Anwar al Awlaki, an American-born Islamic preacher who was a leading figure in Yemen-based Al Qaeda in the Arabian Peninsula.

Awlaki, who U.S. officials said was in correspondence with Major Nidal Hassan, a U.S. Army psychiatrist, before Hassan embarked on a deadly shooting spree at a Texas military base, was killed in a CIA drone strike.

U.S. officials said they had no evidence of threats to the United States following the attacks in Canada. The U.S. Embassy in Ottawa, on lockdown Wednesday, reopened on Thursday.

"At this time, there is no specific reporting to indicate that ongoing events in Canada pose a threat to the United States," said Aaron Bowker, spokesman for U.S. Customs and Border Protection's Buffalo field office.

Law enforcement officials said Wednesday's shooting of a soldier in the Canadian capital appeared to be the act of a single individual, the type of "lone wolf" attack U.S. authorities say it is difficult to defend against.

"It's so simple and unpredictable, it’s hard to know exactly what to do," said Captain Rick Hopkins of the state police in Vermont, which shares a border with Canada.

"You want to do all you can to keep people safe, but it’s very frustrating (that) you can’t point to one thing” that would stymie such an attack, Hopkins said.

(Editing by Howard Goller)