BETHLEHEM (Ma'an) -- Israeli Prime Minister Benjamin Netanyahu on Wednesday warned of "extensive and deeper action" in Gaza as militant groups announced a joint operation room in the enclave.

"We neither chose nor initiated this escalation but if it continues we are prepared for much more extensive and deeper action," Netanyahu said during a tour in Ashkelon, near the Gaza border.

"Whoever intends to attack Israeli citizens needs to know that he will bear the consequences," said the Israeli premier, who is seeking a renewed mandate in Israel's Jan. 22 election.

Opposition leader Shaul Mofaz accused Netanyahu of "stuttering" on Hamas and said he had not been decisive enough during his term in office, Israeli media reported.

Hamas accused Israel of stepping up its air strikes in the Gaza Strip to vent its anger over Qatar's Sheikh Hamad bin Khalifa al-Thani's visit to Gaza on Tuesday and pledged to "continue to hold a gun ... until Palestine is liberated".

Israel has killed four Palestinians, including three Hamas militants, in a series of airstrikes since Tuesday evening. Militants in Gaza have fired 76 rockets into Israel since midnight, an Israeli military spokeswoman told Ma'an.

The head of the al-Nasser Salah al-Din brigades, the military wing of the Popular Resistance Committees, said his group had formed a joint operations room with Hamas' al-Qassam brigades and the armed wing of Islamic Jihad, the al-Quds brigades.

The groups have joined forces "to discuss ways to directly respond jointly to any Israeli attacks against any Palestinians," Abu Suhaib told Ma'an.

"The joint operation room will lead the coming battle in case Israel attacks Palestinians.

"The unity of the resistance bodies hurts the enemy, the Israeli occupation has tried to break down the unity of the resistance and assassinate a number of leaders, however the bodies of resistance gathered their parts immediately and reunited again to protect the Palestinian people," Abu Suhaib said.

Al-Quds brigades spokesman Abu Ahmad said a united response to Israel was important where possible, but said joint actions depended on field conditions.

The al-Qassam brigades and the al-Nasser Salah al-Din brigades attacked Israeli military bases around the Gaza Strip in response to Israel's airstrikes, Hamas' armed wing said in a statement.

The operations were intended to repel Israeli attacks on the West Bank and Gaza "and as a response for the ongoing aggression against Palestinian people," the al-Qassam brigades said.

Gaza's education ministry said six schools have been evacuated in Khan Younis due to Israeli airstrikes in the area, and Israel kept schools shut in communities near the border.