Israel fears that the Islamic militant Hamas is trying to copy the tactics of the Lebanese guerrilla group Hezbollah, which fought Israel to a draw in last summer's war in Lebanon.

Hamas has exploited a period of relative calm to smuggle large numbers of anti-tank missiles and 30 tons of weapons-grade explosives into Gaza, using tunnels under the border with Egypt, Israeli security officials said, speaking on condition of anonymity because they were not authorized to discuss the issue with reporters.

Hamas has also nearly doubled its number of fighters to 10,000, the security officials said.

Hamas dismissed the claims as Israeli propaganda. A spokesman for the group, Islam Shahwan, said the Hamas force has 5,500 fighters.

Hamas formed a coalition with the Fatah movement of moderate Palestinian President Mahmoud Abbas two weeks ago. The new government has said it's ready to consolidate a four-month-old truce, but has stopped short of renouncing violence.

In an interview broadcast Saturday, Israeli Prime Minister Ehud Olmert was asked whether a large-scale Israeli invasion of Gaza was needed to halt an arms buildup in Gaza.

"The question is if it has to be a military operation, if it has to be a military operation by us and if it has to be now," Olmert told Channel 2 TV. "We won't shy away from a military operation if we reach the conclusion, after a thorough check, that it is possible, based on logic and level-headedness and no exaggerations, that there is no better way than this."

Asked whether this was the case now, he said: "This is not the case."

Diplomatic constraints might keep Olmert from ordering a Gaza invasion. The U.S. is trying to revive Mideast peacemaking by winning greater involvement of moderate Arab states, and a large-scale Israeli military operation could sabotage such efforts.

Olmert is also still fighting off criticism that he embarked on the war against Hezbollah hastily. He might also be concerned about getting entangled in fighting in crowded Gaza; previous operations have caused many casualties, drawing international condemnation but yielding few achievements for Israel.

Still, senior military commanders have repeatedly expressed concern about the arms buildup.

Last month, Israel's southern commander, Maj. Gen. Yoav Galant, warned that Iran is helping Hamas upgrade its fighting strength by providing technology, funding and direct military training.

Galant said the training and technology has enabled Hamas to grow from a ragtag militia into a well-organized group resembling an army ¡ª complete with battalions, companies, platoons and special forces for surveillance, snipers and explosive experts.

Also, the head of Israel's Shin Bet internal security agency, Yuval Diskin, has said that dozens of Hamas operatives are going to Iran for training. He gave no proof for the allegation.

Shahwan, the Hamas spokesman, said members of the Palestinian security forces are being trained in Islamic countries, including Iran, but that not all are members of the Hamas unit.