NEW YORK - Police responded to two false bomb alarms in New York and at nearby Newark airport Monday as jitters grew over security during the busy end-of-year holiday season.

Part of Newark Liberty International Airport in New Jersey was briefly shut down after discovery of a suspicious package that turned out to contain a harmless computer.

A few hours later, police responding to a suspicious vehicle closed off part of a street joining famed 5th Avenue in Manhattan, near the tourist magnet of the Metropolitan Museum of Art.

A Labrador dog from the bomb sniffing unit was brought to inspect a silver minivan and its contents, which were laid out in the middle of the closed-off street.

After declaring an all clear, police reopened the area to traffic.

The Christmas and New Year's period is the busiest for travel in the country.

Last week, President Barack Obama's top counter-terrorism advisor said "appropriate resources" had been deployed to ensure security.

John Brennan warned however that Al-Qaeda was always looking to strike the United States and that the counter-terrorism community was particularly vigilant because of increased holiday season travel.

In Britain on Monday, police arrested 12 men on suspicion of plotting an Al-Qaeda linked terrorist act.

New York has been the target of multiple bomb plots since the September 11, 2001, carnage at the World Trade Center. However, police also respond repeatedly to false alarms.

One jihadist activist monitored by the private SITE Intelligence Group suggested to others online that they call in bogus bomb threats to the police or media offices.

The activist, a member of the Shumukh al-Islam forum, said "they will consider every communication such as this as a real report, for they are cowards and they fear that all reports are true," SITE reported.