But his remarks were contradicted by the country's police spokesman, who said no terror attack was expected in Turkey.
The reason for the discrepancy was not clear, but authorities might be worried that alarming statements could scare away tourists and cause panic among the public.
Sammaz Demirtas, the deputy chief of police for Istanbul, told Vatan newspaper on Friday that he expected an attack within four months and said the police in Istanbul was monitoring some 1000 people believed to have links to al-Qaida.
But Ismail Caliskan, police spokesman for the whole country, responding to a question about Demirtas's statement, told a news conference that no terror attack was expected.
"As the police headquarters, we do not anticipate any (terror) action against Turkey. Turkey is a safe country," Caliskan said.
However, Caliskan confirmed that police were following radical Islamic and separatist groups.
Police had arrested over 60 people
in connection with 2003 attacks
"Our security forces are continuing their work uninterrupted at the highest level against all separatist and radical religious groups," Caliskan said.
Istanbul was the target of terror attacks in November 2003, when Islamic militants, affiliated with al-Qaida, bombed a London-based bank, the British Consulate and two
synagogues, killing 58 people and wounding hundreds more.
Turkey arrested more than 60 people in connection with the attacks, and security officials say they have regularly foiled would-be attackers' plans since.
"I expect a new attack in Turkey before November," Demirtas told Vatan newspaper. "Eighty percent of intelligence police officers are working on groups with a religious tint."
Foreign trained militants
"We are watching 1000 people who we have established ties with al-Qaida," he said.
"We are watching 1000 people who we have established ties with al-Qaida"
deputy police chief, Istanbul
Police officials have said they are focusing on hundreds of Turks who fought in Chechnya, Afghanistan or Bosnia, some as members of al-Qaida. Police fear Turks who fought abroad were trained or influenced by radical groups like al-Qaida.
Some of the suspects behind Istanbul bombings in 2003 have also fought in such countries.
Meanwhile, police in Istanbul have stepped up security at subway and tram stations since the London bombings, deploying plain-clothed officers and sniffer dogs trained to find explosives. More police officers have been assigned to guard the stations.
Authorities in Istanbul also plan to increase the number of closed-circuit television cameras in the city.