* Two Israelis, two Palestinians reported killed
* Hamas supporters hold street celebration
* Israeli tanks enter Gaza firing shells
(Updates with fighting abated but tanks still in place)
By Nidal al-Mughrabi
GAZA, March 26 (Reuters) - Israeli tanks advanced into the Gaza Strip on Friday after the worst clash with Palestinian fighters in 14 months killed two on each side, and Palestinian sources reported more casualties in the fighting.
They said five Israeli tanks and two armoured bulldozers advanced from the east firing shells at targets near the town of Khan Younis in the centre of the narrow coastal enclave.
The Gaza-based militant group Popular Resistance Committees confirmed one of its fighters was critically wounded by shelling. Palestinian sources reported Israeli helicopters and unmanned military drones in the skies.
Witnesses near the scene said the sounds of gunfire abated an hour before midnight (2100 GMT) but the tanks were still in place firing occasional rounds. Residents were fearful of a bigger Israeli incursion, and evacuation warnings were given.
The Israeli army earlier said an officer and a conscript were killed when Palestinian gunmen fired on an Israeli military patrol inside the Strip. Two soldiers were wounded and two Palestinian fighters also died in that clash, it said.
Palestinian officials did not immediately confirm the two deaths but said at least five Palestinians, one a 10-year-old boy, were wounded according to Gaza hospital officials.
"I think it’s true to say that this is one of the fiercest days we have had since operation Cast Lead happened," Israeli army spokeswoman Avital Leibovich said.
It was a "tragic and a painful" incident in a zone where there is "an everyday war", with Palestinian gunmen planting explosives near the fence and frequent cross-border fire at ranges of a couple of hundred metres.
Israeli forces "have to operate from both sides of the fence in order to have a maximum defensive system", she said.
The clash did not appear to be directly linked to the current diplomatic impasse between Israel, the Palestinians and the United States over Israeli settlement of occupied West Bank land and stalled efforts to relaunch peace talks.
Israel unilaterally withdrew from Gaza in 2005.
HAMAS SUPPORTERS CELEBRATE
The militant Islamist group Hamas which has ruled the enclave since 2007 said its men had fired on Israeli soldiers who crossed into the Strip.
Hundreds of Hamas supporters took to the streets of Jabalya refugee camp in the northern Gaza Strip to celebrate the killing of the two Israeli soldiers, led by senior Hamas lawamaker Mushir Al-Masri, who praised the battle.
"Entering Gaza is not a picnic," he said. "The Zionists cannot come in anytime they wish and leave anytime and however they want," Masri told the crowd. "The Qassam Brigades (Hamas’s armed wing) were ready and taught them a lessson and they should not repeat such a foolish act,"
Hamas had largely held its fire since a costly three-week war with Israel in the opening days of 2009 in which some 1,400 Palestinians, mainly civilians, and 13 Israelis, mainly soldiers, were killed.
But smaller factions have violated the de facto truce by firing rockets and mortars into neighbouring Israeli territory.
Israel said it was holding Hamas accountable for the violence, which made further retaliatory action likely. "Hamas is accountable for any activity that takes place from Gaza to Israel," an Israeli military spokeswoman told reporters.
Two other groups, Islamic Jihad and al-Aqsa Martyrs Brigades, said their men also took part in the fighting.
Israeli armoured pursuits into Gaza are not unusual. The Israelis try to maintain a buffer zone within the border fence and keep it off-limits to Palestinian fighters.
Statements by Hamas, Islamic Jihad and al-Aqsa Martyrs Brigades did not make clear who carried out the initial ambush on the Israeli patrol. There have been calls from militants for an uprising to resist Israeli settlement in East Jerusalem.
Tensions have run high along the Gaza frontier this month, with Israel launching repeated air strikes in response to Palestinian rocket attacks, one of which killed a Thai worker in a kibbutz farm last week.
An Israeli soldier was accidentally shot dead by comrades earlier this month as they rushed to intercept three Palestinian border-jumpers, who were later found to have been going in search of work in Israel.
The last time an Israeli soldier was killed on the Gaza border was 9 days after the offensive ended in January 2009, when a bomb planted by Palestinian gunmen exploded. (Writing by Douglas Hamilton; editing by Elizabeth Fullerton)