(Adds U.S. State Department comment, paragraphs 13-16)
By Yara Bayoumy
DUBAI, April 16 (Reuters) - The leader of al Qaeda's wing in Yemen has vowed to attack the United States, in a video apparently showing a gathering of the group celebrating a mass jailbreak of fighters.
In February, attackers mounted a bomb, grenade and gun assault on the main prison in Sanaa in which 29 inmates, including 19 jailed for terrorism-related crimes, escaped.
The 15-minute video, dated March 2014 and posted on a website used by Islamists, shows masked men waving al Qaeda's black flag and celebrating the arrival of the freed prisoners.
Its authenticity could not be independently verified.
"The Crusader enemy, dear brothers, still possesses cards which he moves around. We have to remember that we are always fighting the biggest enemy," says a man speaking in the open in a mountainous area, whom the video identifies as its leader Nasser al-Wuhaishi.
"We have to remove the cross, (and) the bearer of the cross, America," said Wuhaishi, who appeared with his face uncovered, wearing a T-shirt and sporting a dagger common among Yemeni men.
The video, entitled "Images from the reception of the freed prisoners from Sanaa's central prison," included testimony from fighters involved in the jailbreak.
"We planned that we would need 10 hand grenades," said a man identified as Munir al-Bouni.
Another man said: "Once you got out, you turned right and the guys were waiting."
LARGEST GATHERING IN YEMEN
Abdulrazzaq al-Jamal, a journalist who has interviewed members of al Qaeda in the Arabian Peninsula (AQAP), said it was the largest known gathering of al Qaeda in Yemen.
"It included about 400 people, and these are not all the members of the group. This gathering is a confirmation that al Qaeda is gaining strength and influence," he told Reuters.
Formed in 2009, AQAP has attacked military targets, tourists and diplomats in Yemen and taken over territory for long periods.
A U.S. State Department spokeswoman said in response to the video that it was "in no way breaking news that AQAP is a significant threat to the United States, the people of Yemen, to other people in the region and around the world."
Since 2009, the group had made several attempts to attack the United States and had carried out a number of attacks inside Yemen, Marie Harf told a regular briefing.
"I don't think we can make generalizations about their strength based on one video, quite frankly. We know they've been gaining in strength. ... So, I don't think this increases our concern, because quite frankly, our concern was already incredibly high."
Harf said it was interesting that Wuhaishi was not only head of AQAP, but was now number two of core al Qaeda, showing that the main body had decentralized as a result of U.S. successes against it in Afghanistan and Pakistan.
The video showed men arriving in white pickup trucks and others greeting Wuhaishi. Others carried rifles, and some sat on the ground drinking juice. Some spoke or sang through microphones and loudspeakers.
U.S. drone attacks have killed several suspected AQAP figures, including Anwar al-Awlaki, a U.S.-born Islamist cleric accused of links to a plot to bomb a Detroit-bound airliner in 2009 and U.S. cargo planes in 2010.
"Al Qaeda will gain strength as there continues to be division and weakness in security performance," said Ali al-Sarari, political adviser to Prime Minister Mohammed Salem Basindwa.
"The last appearance (the video) shows that Yemen is confronting a dangerous challenge. And if the security performance continues in this manner, then Yemen will face an even bigger failure than what it has faced in the past." (Reporting and writing by Yara Bayoumy; Additional reporting by David Brunnstrom in Washington; editing by Sami Aboudi, Andrew Roche and Tom Brown)