Turkey’s leaders are prepared to use the armed forces against protesters if they consider it necessary, the deputy prime minister said Monday, raising the threat of military intervention for the first time during the ongoing unrest, in a country that has only recently moved away from its long history of army coups.

With protesters and police continuing to skirmish in Istanbul and the capital, Ankara, the warning from Bulent Arinc was a significant rhetorical escalation in the crackdown on anti-government demonstrations that began May 31. And the assertion that the civilian government retained control of the armed forces, though not unexpected, was notable.

“The police and security forces are in charge. If this is not enough, the gendarmerie will be in charge,” Arinc told state-run TRT television in Ankara. “And if this is still not enough . . . we can use the Turkish armed forces.”

Arinc was responding to a question about the use in recent days of the gendarmerie, a military-type security force that is under a separate civilian command.

Prime Minister Recep Tayyip Erdogan has taken major steps in his 10 years in power to reform the military and forestall the possibility that it might step in during periods of civil unrest, as it did four times in the second half of the 20th century. Erdogan’s efforts at the time were supported by many of the protesters who have now taken to the streets to fight what they say is a steady erosion of personal liberties under his government. But even as he attempted to curb the military, many critics were suspicious that he was doing it simply to expand his own power.

For Erdogan’s opponents, Monday’s announcement was a ratification of that fear.

“This shows the depth of the panic, coupled with their determination to crush this movement at any cost,” said Osman Faruk Logoglu, the deputy chairman of the opposition Republican People’s Party, whose regional offices in Istanbul were vandalized Sunday.

But one former military official said the situation was unlikely to deteriorate so much that Erdogan would give orders to roll out the army. “I don’t really believe it will escalate to that situation right now,” retired Maj.Gen. Armagan Kuloglu said.

In Washington, State Department spokeswoman Jen Psaki said Monday that Secretary of State John F. Kerry spoke to Turkish Foreign Minister Ahmet Davotoglu on Saturday, mostly about Syria. She would not address whether the United States is disappointed with the Turkish government.

“We are focused on calling for calm, on calling for restraint. We remain a close ally of Turkey,” Psaki said, but “we haven’t held back” in airing concerns about the Erdogan government’s behavior.

Five major labor unions called a one-day strike Monday and tried to march to Istanbul’s Taksim Square, but lines of riot police stopped them. Earlier in the day, the square had reopened, two days after security forces pushed out the two-week-old occupation in adjoining Gezi Park, one of the biggest challenges to Erdogan’s rule.

The park was still closed to the public, as swarms of workers planted new grass and dozens of trees in an apparent effort to prove the environmental bona fides of the government. Video of the new plantings was repeatedly broadcast on state-run television.

The protests were sparked by plans to raze Gezi Park and build a replica of an Ottoman-era barracks that once stood at the site. After police responded May 31 to a small group of peaceful protesters with tear gas and water cannons, the demonstrations quickly expanded to include broader complaints about restrictions on personal liberties.

With the Saturday crackdown, Istanbul’s protests — once largely confined to Taksim Square and Gezi Park — have spread across the vast city of 13 million. Police forces have targeted doctors, lawyers and journalists, and Erdogan has compared people who aid the protesters to terrorists.

Many neighborhoods were tense Monday, after overnight clashes with police, but the situation was quieter at night, with police using tear gas and water cannons in only a few areas.

The crackdown has drawn international condemnation. German Chancellor Angela Merkel said Monday that she was “appalled, like many others,” at Turkey’s treatment of the protesters.

Anne Gearan in Washington contributed to this report.