Smoke rises from a publishing house in the center of Grozny, Russia, early Thursday, Dec. 4, 2014.

The violence erupted hours before Russian President Vladimir Putin was to make his annual state of the nation address in Moscow.

Security officials said militants traveling in three cars entered the republic's capital, Grozny at 1 a.m. local time, killing three traffic police at a checkpoint. The Moscow-based National Anti-Terrorist Committee, a federal agency, said the militants then occupied a multi-story Press House in central Grozny which was later destroyed by fire, killing six gunmen. An unspecified number of law enforcement officers also were killed, authorities said.

The Anti-Terrorist Committee said more gunmen had been found in a city school and an operation was underway to "liquidate" them. There was no indication that any children were in the school in the early morning.

The mood was tense in Grozny on Thursday with artillery fire heard in the background and the area about the Press House and the school building cordoned off.

Although unrest is common across the North Caucasus, forceful security measures adopted by Chechen leader Ramzan Kadyrov have spared Grozny significant violence for several years. That has allowed Putin to claim success in subduing an Islamic insurgency in Chechnya after years of war.

Dmitry Trenin, who heads the Carnegie Moscow Center, wrote in a Twitter post that "the night attack in Grozny looks senseless, except as an attempt to embarrass Putin hours before his annual address to parliament." Putin already was under pressure to reassure Russians as fears grow over soaring inflation and a plummeting ruble.

An Associated Press reporter saw the building — a publishing house — in flames and heard the sound of heavy-caliber gunfire before dawn, several hours after the unrest erupted. The AP reporter also saw the body of someone in civilian clothing in the street near the publishing house as fighting continued, but it was not clear how and when the person had been killed.

The Anti-Terrorist Committee announced that it had imposed a counterterrorism regime on the center of Grozny. This officially allows heightened security measures to be enforced, and typically indicates the imminent use of heavy force.

Life News, a news outlet believed to have links to Russian security services, cited law enforcement officials as saying about 15 people seized three cars late Wednesday in the village of Shalazhi and drove to Grozny, some 50 kilometers (30 miles) away.

Kadyrov said on his Instagram account, which he uses to issue public statements, that the traffic police officers were shot dead as they attempted to stop the cars carrying the gunmen.

Kadyrov said the situation was calm and that all essential public services were operating, but he urged Grozny residents to be cautious.

"I ask residents in areas where (security) operations are being carried out to abide by safety measures, and not to go out onto the streets without cause or to go near their windows," he wrote. "All the talk about the city being under the control of the military is absolutely false."

In a message posted several hours later, Kadyrov said that six militants were killed in the standoff at the publishing house.

"Not one bandit managed to get out. I directly ran the operation myself," he wrote.

Kadyrov posted a picture showing the lower half of an apparently dead gunman lying beside a rifle, but it was not immediately clear if it showed one of the presumed attackers.

The Kavkaz Center website, a mouthpiece for Islamic militant groups operating in Russia's North Caucasus, carried a link to a video message by an individual claiming responsibility for the attacks. The man in the video claimed to be operating under orders from Chechen Islamist leader Aslan Byutukayev, known to his followers as Emir Khamzat.

The video could not immediately be verified.

A few years after the collapse of the Soviet Union, Chechnya was plunged into a full-scale war when separatist rebels pursued independence for the republic. The violence was largely confined to that small republic, but rebels ventured into other parts of Russia.

A fragile peace settlement was reached with Moscow until 1999, when an insurgency movement increasingly inspired by radical Islamist ideas reignited the conflict. A military crackdown succeeded by years of aggressive rule by Kadyrov has quietened the region, pushing unrest to neighboring provinces.

Kadyrov has been widely denounced for human rights abuses, including allegations of killing opponents. He has also imposed some Islamic restrictions on the region, including mandatory public headscarves for women.