RAMALLAH/JERUSALEM (Reuters) - Israel has ordered its military to prepare for an "extended stay" in parts of the occupied West Bank as it steps up operations against Palestinian militant groups, the Israeli defence minister said on Sunday.

Prime Minister Benjamin Netanyahu instructed the military to carry out an "intensive" operation in the West Bank following explosions on buses close to Tel Aviv on Thursday, in what Netanyahu's office described as an attempted mass attack. No casualties were reported.

Israel's military has been conducting a large-scale military operation in the West Bank over the past month that it says is targeting militants. Tens of thousands of Palestinians have been forced to leave their homes in West Bank refugee camps, while homes and infrastructure have been demolished.

In a statement on Sunday, Defence Minister Israel Katz said he had ordered military forces to expand operations in the Palestinian refugee camps of Jenin, Tulkarem and Nur al-Shams in the northern West Bank in order to dismantle militants' infrastructure.

He said 40,000 Palestinians had left the camps.

Tanks will be deployed to Jenin as part of the offensive, the military statement added, in the first deployment of tanks by Israel in the northern part of the Israeli-occupied West Bank for more than 20 years.

Nabil Abu Rudeineh, a spokesperson for Palestinian President Mahmoud Abbas, condemned the decision to deploy tanks in the northern West Bank.

"This is a dangerous Israeli escalation that will not lead to stability or calm, and we warn of this dangerous escalation," he said.

In his statement, Katz said the U.N.'s agency for Palestinian refugees, UNRWA, has been told to halt its activity in the camps. The agency could not immediately be reached to comment.

No group has claimed responsibility for the Tel Aviv bus blasts, which came amid a fragile ceasefire in Gaza between the Palestinian militant group Hamas and Israel after almost 16 months of war.

The Gaza ceasefire has held since it was implemented on January 19 despite accusations traded by Israel and Hamas of violations.

(Reporting by Ali Sawafta in Ramallah, Emily Rose in Jerusalem, Clauda Tanios in Dubai and Muhammad Al Gebaly in Cairo; Editing by Helen Popper)