(Adds Palestinians released, militants train for possible incursion)

By Nidal al-Mughrabi

GAZA, Sept 15 (Reuters) - A small Israeli force pushed into the northern Gaza Strip on Saturday, clashing with militants and bulldozing farm land, the army and residents said.

Local medics said a 17-year-old Palestinian was in serious condition after being shot in his stomach in northern Gaza. The army said it had no reports of injuries.

An army spokesman said Israeli forces conducted "routine activity" against what it called terror threats in the north of the coastal enclave. Troops earlier detained nine Palestinians for questioning but they were later released.

Militants in northern Gaza fired a rocket propelled grenade at the Israelis and exchanged fire with soldiers, he said.

Local residents said a small group of armoured vehicles bulldozed farm land in northern Gaza. A small force also entered southern Gaza, according to residents, but the army said there was no activity in the area.

The Israeli army declined to comment on whether it used bulldozers but said it sometimes used engineering vehicles during operations aimed at curbing militant activity.

Tensions between the Jewish state and Hamas, which seized control of Gaza in June, have increased sharply after a makeshift rocket fired into Israel by militants wounded at least 35 Israeli army conscripts this week.

The Israeli army frequently carries out operations in border areas to try to curb rocket attacks from Gaza. Israel has so far opted against a major offensive in the territory but this week's rocket attack rekindled calls for tougher action.

Scores of masked Hamas security force members fired automatic rifles and laid explosives during training for a possible broader Israeli offensive in Gaza late on Friday. The Hamas-aligned Popular Resistance Committees also held a training session in Gaza on Saturday.

Militants from major groups said they were on high alert in case Israel launched a major incursion into Gaza, and were deploying hundreds of fighters near the border every night.

Separately, the Israeli army arrested 12 protesters at a rally near the city of Nablus in the occupied West Bank after they hurled stones at soldiers and tried to force their way through a checkpoint, an army spokesman said.

Two of those arrested were Palestinian and the rest were Israelis and foreigners. One protester was lightly injured and taken to a hospital, the spokesman said, adding that about 40 people attended the rally.

An Israeli newspaper Web site said the protesters were demonstrating against a planned road block in the area.

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