Israeli police said Friday that at least 12 people were in custody on suspicion of starting huge fires that have forced thousands of people to evacuate their homes and left widespread damage across the country.

Some of the fires, which started Monday night, were still burning Friday, and numerous countries, including Russia, Turkey, Jordan and Egypt, had sent emergency crews and firefighting planes to help Israel tackle the blazes. The Palestinian Authority sent four firefighting teams.

Israeli news media reported that the largest firefighting plane, the Supertanker, was on its way from the United States, and several American fire crews also were expected to arrive.

Officials and politicians said that at least half of the fires, which raged from the Jerusalem area to the north of the country, had been started intentionally. They were quick to label the fires a new form of terrorism, but police have not made public the identities of the people arrested.

Speaking to reporters, Israeli Prime Minister Benjamin Netanyahu said Israel was dealing with a new form of attack, pyro-terrorism. He said there was proof that at least some of the fires were started intentionally, indicating that those most likely to blame were Palestinians with nationalist hostilities against Israel.

“We can’t say if it is something that is organized, but we are seeing cells starting these fires,” he said. He refused to expand further on the theory but said the security services had hard evidence.

“Like all crimes, the most important thing is the punishment. There is a price to pay for these crimes,” he said.

Arab Israeli politicians responded that the government was taking advantage of the situation to incite anger toward Arab Israelis and Palestinians. They said many of the areas damaged by the fires included Arab communities.

“It is unfortunate when elected officials choose to enhance the division in Israeli society and add fuel to the fire,” said Jamal Zahalka, an Arab Israeli member of Israel’s parliament. “The Israeli media has neglected to mention that various Arab cities have also caught fire.”

On Thursday, more than 60,000 people in the port city of Haifa were evacuated. By Friday morning, however, the fires seemed under control and people slowly began to return to their homes.

More than 100 people have been injured, mostly from smoke inhalation, but no fatalities have been reported.