A group within Turkey's military has engaged in what appeared to be an attempted coup, the prime minister said, with military jets flying over the capital and reports of vehicles blocking two major bridges in Istanbul.

The military said in a statement it seized control "to reinstall the constitutional order, democracy, human rights and freedoms, to ensure that the rule of law once again reigns in the country, for the law and order to be reinstated."

The statement went on to say that "all international agreements and commitments will remain. We pledge that good relations with all world countries will continue."

An announcer on TRT television said members of the military had entered the station and demanded the network broadcast additional information, including that a curfew was in place and that martial law had been imposed.

The announcement said a "peace council" had been established to maintain order and that a new constitution would be prepared as soon as possible. TRT was later taken off the air.

CBS News foreign correspondent Holly Williams reported from Istanbul that Turkish President Recep Tayyip Erdogan has spent time trying to decrease the influence of the military, in effect attempting to "defang" the armed forces.

Erdogan spoke with a private Turkish TV network on a live video-streaming app, telling the public that he and his government remain in control. He urged the Turkish people to rise up against the coup attempt.

"I urge the Turkish people to convene at public squares and airports," Erdogan said. "There is no power higher than the power of the people. Let them do what they will at public squares and airports."

He continued, "The chain of command has been violated. This is a step against the higher ranks by their superiors. The judiciary will swiftly respond to this attack. I invite the people to public squares."

After Erdogan's appearance, live video appeared on social media showing crowds of people gathering in public squares.

An extremely polarizing figure, Erdogan became president in 2014. Prior to that, he was prime minister and the mayor of Istanbul.

"People either love him or loath him. There is really nothing in between," Williams told CBSN.

Williams said soldiers and military vehicles were blocking traffic on the Bosporus and Fatih Sultan Mehmet bridges in Istanbul.

A State Department source and a U.S. intelligence official confirmed to CBS News a change of command was expected to take place next month for Turkey's military. There was speculation that some in the military did not like who Erdogan was going to place in control.

A source in Erdogan's office responded to CBS News by saying the appointments were routine and happen every year. The source said the coup attempt was launched by the Gulen Movement, named for reclusive preacher Fethullah Gulen.

Several known Gulenists have hinted about the possibility of a coup in recent months, according to the source.

A senior U.S. intelligence official told CBS News senior investigative producer Pat Milton that the intelligence community was closely monitoring the situation, but that it was still very fluid.

Secretary of State John Kerry, in Moscow to meet with the Russian foreign minister about the situation in Syria, said he was also watching developments from Turkey.

Kerry said he could not comment further but added, "I hope there will be peace and stability and continuity within Turkey."

The State Department told U.S. citizens in Turkey to "shelter in place and stay indoors," and to notify their friends and family that they were safe.

As CBS News foreign affairs correspondent Margaret Brennan reported from the State Department, Turkey is a NATO member, which binds the U.S. to help protect it. Turkey has also been an important regional ally in the U.S.-led coalition fighting the Islamic State of Iraq and Syria (ISIS).

There are more than 2,000 American troops at Incirlik Air Base in southern Turkey, near the Syrian border. Keeping those troops in place is a priority for the U.S., which launches airstrikes against ISIS from Incirlik.

The U.S. has recently been very concerned about Erdogan's increasingly authoritarian tendencies, according to Brennan. He has been silencing critics, journalists and trying to crush Kurds pushing for independence.

Two weeks ago, 44 people were killed in a triple suicide bombing at Istanbul's airport. Turkish officials believe ISIS was responsible for the attack.

Without offering specifics, Prime Minister Binali Yilidirm said on Twitter sieges were underway at some important buildings. He urged people to remain calm and said acts against democracy would not be tolerated.

The Turkish military chief of staff was among those being held hostage in Ankara, the state-run Anadolu news agency said.

Yildirim told NTV television: "it is correct that there was an attempt," when asked if there was a coup. He didn't provide details, but said Turkey would never allow any "initiative that would interrupt democracy."

"We are focusing on the possibility of an attempt (coup)," Yildirim said. "There was an illegal act by a group within the military that was acting out of the chain of military command. Our people should know that we will not allow any activity that would harm democracy."

Yildirim said those responsible would pay the highest price, Reuters reported.

Earlier, military jets were heard flying over the capital, Ankara. Media reports said ambulances were seen in front of the Turkey's military headquarters with helicopters flying overhead.

Gunfire was heard near Istanbul's police headquarters, according to NTV.

All flights from Istanbul's Ataturk Airport have been cancelled, a witness citing a pilot at the airport told Reuters. Photos on social media showed what appeared to be a military tank blocking an entrance to the airport.