(Adds Hamas comment paragraph 5)

By Nidal al-Mughrabi

GAZA, June 5 (Reuters) - Rival Palestinian forces clashed in the Gaza Strip on Tuesday near the strategic Karni commercial crossing in the most serious flare-up in factional fighting in more than two weeks.

An officer with President Mahmoud Abbas's Presidential Guard said a "large number" of Hamas fighters tried to infiltrate a key Presidential Guard position near the crossing, wounding at least one guard member.

The officer and a senior Western security source said the Hamas fighters were pushed back after a nearly three-hour gun battle with the Presidential Guard, a Fatah-dominated force that is loyal to Abbas and receives U.S. backing.

Hamas, which leads a Palestinian unity government with Abbas's Fatah faction, confirmed the fighting near Karni but said the Presidential Guard initiated the exchange and that no Hamas fighters were hurt.

Abu Ubaida, a spokesman for Hamas's armed wing, later denied that members of the group were directly involved in the clash. "Our men did not fire. There has been no attempt at all by Hamas fighters to take over or to infiltrate any position of the Presidential Guard," he said.

Another senior Hamas source said the group's gunmen had been in the area to monitor Israeli forces near the Gaza border when they came under fire.

The senior Western security source, who works closely with the Presidential Guard, said between 50 and 100 Hamas fighters took part in the early morning assault, but those figures could not be independently confirmed.

"They (the Hamas fighters) tried to take over the crossing but were repulsed," the Western security source said.

Washington recently approved a $59.4 million security programme which includes $43.4 million to "transform and strengthen" Abbas's presidential guard by providing its members with non-lethal equipment and training.

The United States and Israel say Hamas's armed wing and Executive Force receive funding and arms from Iran and other Islamist allies.

The Western security source said Hamas attacked the same position, a Presidential Guard training base next to Karni, on May 15 at the height of last month's factional fighting between secular Fatah and Hamas Islamists.

Virtually all trade with Gaza has to pass through Karni.

Tuesday's flare-up was the most serious since a shaky ceasefire between Fatah and Hamas took hold two weeks ago after a wave of factional fighting killed some 50 Palestinians. (Additional reporting by Adam Entous)

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