Bombs exploded simultaneously at two Istanbul hotels, killing two people and injuring 11 in what the city's police chief said was a "terrorist attack."
An Islamic militant group posted on the Internet a claim of responsibility for bombings.
One of the hotels was only a few miles from where the U.S. Olympic men's basketball team is staying during the final stop of its pre-Olympic tour. The team toured Topkapi Palace on Monday, guarded by a large contingent of police.
Two bombs also exploded within a half hour of each other at a gas plant in the outskirts of the city, causing damage but no casualties, Tayfun Demiroren, an official at the at the liquefied petroleum gas plant, said.
There were no immediate claims of responsibility in the bombings, but a police official, speaking on condition of anonymity, said Kurdish rebels could be behind the attacks, which came hours after the arrest of four Kurdish militants in the city and after earlier threats by a Kurdish rebel group to strike Turkey's tourist trade.
The workers at the Pars hotel received a warning of a bomb in a room, and 10 minutes later about 2 a.m. an explosion killed two people, an Iranian and a Turk, the Turkish Anatolia news agency said. The hotel is in an area catering to East European tourists.
At the Star Holiday Hotel in an area where the Ottoman Topkapi Palace is located, glass and chunks of concrete littered the streets. "It appears to be a terrorist attack," Police Chief Celalettin Cerrah told Anatolia.
"There was a huge explosion and the glass started shattering," said Umut Akgul, who had been visiting a friend who works at the Star Holiday Hotel at the time of the blast.
Akgul said he ran to the back of the three-story hotel and started to help evacuate tourists after the explosion, which ripped off the exterior walls of the top two floors.
In all, nine people were injured in the hotel blasts.
Kurdish rebels, battling Turkish troops in the southeast for autonomy, have intensified attacks lately.
The rebel group, KONGRA-GEL, had threatened to target the country's tourism industry and infrastructure, and broke a unilateral cease-fire on June 1, saying Turkey did not respond in kind.
Security concerns in Turkey have been heightened since last November, when four suicide truck bombings blamed on al Qaeda killed more than 60 people in Istanbul.
A spokesman for the U.S. Olympic team was not aware of the bombings until being contacted by a reporter.