By Nidal al-Mughrabi

GAZA, Jan 12 (Reuters) - An Israeli air strike killed at least two Hamas gunmen and critically wounded another in the southern Gaza Strip on Saturday, local medical workers and officials said.

Hamas said the missile hit one of their military positions in the town of Khan Younis and the two dead men belonged to the organisation's armed wing.

An Israeli army spokeswoman confirmed the air strike and said it was in response to the frequent firing of rockets and mortar bombs by militants into nearby Israeli towns.

The spokeswoman said since the start of the year 215 launchings had been counted. Israel often targets Gaza militants in air and ground attacks as it aims to thwart the firings, which spread panic, but rarely result in any injuries.

In another development in the coastal territory, Hamas security officials said they arrested a Palestinian who was set to detonate a bomb at a gathering hosted by Hamas leader Ismail Haniyeh for returning Haj pilgrims on Saturday afternoon.

Hamas official Ehab Alshsain said security men caught a suspect with 4 kg (9 lb) of TNT ready to be exploded by cellular phone at rally where Haniyeh welcomed home hundreds of pilgrims.

"The investigation is taking several directions (to discover) whether the aim was to assassinate Haniyeh, or to kill as many people as possible," Alghsain said.

Another security official accused Fatah of being behind the plot. A Fatah official in the West Bank rejected the allegation as "fabrications and lies".

"Since Hamas stole the authority in the Gaza Strip it has only been practicing lies and deception," Fatah's Ziad abu al-Enain said in Ramallah.

Hamas Islamists seized control of the Gaza Strip last June after routing the forces of secular Fatah which is led by Palestinian President Mahmoud Abbas. Abbas, whose supporters control the West Bank, dismissed a Hamas-led unity government, sealing a division of Palestinians in the two territories.

Earlier, Palestinian gunmen stormed the American International School in the Gaza Strip, smashing windows, seizing computers and torching buses but causing no injuries, security sources said.

In a leaflet left behind after the attack, the gunmen claimed affiliation with al Qaeda and vowed to attack other symbols of "immorality" throughout the coastal territory.

A series of attacks over the past year in Gaza have been attributed to Islamist groups that say they are followers of al Qaeda.

The school is a private, Palestinian institution that is part of an association of "American Schools" in the Middle East. The curriculum includes English and Arabic studies. None of the teachers is American.

Hamas's interior ministry said in a statement: "An investigation was immediately launched into the attack and we will pursue and bring the attackers to justice regardless of their motives."

The school has been a frequent target for militants. Three days ago, gunmen claiming ties to al Qaeda fired a rocket-propelled grenade at the building during a visit to the region by U.S. President George W. Bush.

In 2006, two teachers were kidnapped from the school by gunmen, prompting all international workers to leave. (Writing by Ori Lewis and Ari Rabinovitch; editing by Philippa Fletcher)

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