BRUSSELS (AP) — NATO Secretary-General Jens Stoltenberg warned Russia Friday that any attempt to invade Ukraine would have costs, as concern mounts about a Russian military buildup near its former Soviet neighbor’s borders.

Ukraine says Moscow kept about 90,000 troops near their common border following massive war games in western Russia earlier this year. The Ukrainian Defense Ministry said units of the Russian 41st army remain near Yelnya, about 260 kilometers (160 miles) north of the border.

Moscow denies that it’s planning any invasion and refuses to provide details about troop movements on its own territory.

“If Russia uses force against Ukraine that will have costs, that would have consequences,” Stoltenberg said, ahead of a meeting of the 30-nation military organization’s foreign ministers in Latvia Nov. 30-Dec. 1, where Russia’s activities will be high on the agenda. He did not say what those costs would be.

“This is the second time this year that Russia has amassed a large and unusual concentration of forces in the region,” Stoltenberg told reporters. He said it includes tanks, artillery, armored units, drones, and electronic warfare systems, as well as combat-ready troops.

“This military buildup is unprovoked and unexplained. It raises tensions and it risks miscalculations,” Stoltenberg said. He conceded that “there is no certainty about the intentions of Russia” but said that “this is a military buildup by a country that has invaded Ukraine before.”

Russia annexed Ukraine’s Crimean Peninsula in 2014 after the country’s Moscow-friendly president was driven from power by mass protests. Weeks later, Russia threw its weight behind a separatist insurgency that broke out in Ukraine’s east.

Ukraine and the West accused Russia of sending its troops and weapons to back the rebels. Moscow denied that, saying that Russians who joined the separatists were volunteers. More than 14,000 people have died in the fighting that devastated Ukraine’s eastern industrial heartland known as Donbas.

A 2015 peace agreement brokered by France and Germany helped end large-scale battles, but efforts to reach a political settlement have failed and sporadic skirmishes have continued along the tense line of contact.

NATO would not be able to provide Ukraine with any substantial military support in time to make a difference against Russian forces, so economic measures like Western sanctions are more likely to be used to inflict a financial cost on Moscow.

In Washington, the State Department’s top official for European and Eurasian affairs, Karen Donfried, told reporters that assessments of Russia’s military buildup near Ukraine will be on the agenda next week when Secretary of State Antony Blinken visits Latvia, and then travels to Sweden for a meeting of the Organization for Security and Cooperation in Europe. She gave no indication whether Blinken would propose new U.S. or allied actions.

“All options are on the table,” Donfried said, “and there’s a toolkit that includes a whole range of options. What we’re doing now is monitoring the region closely, consult with our allies and partners on how do we deter Russian action, and ideally that is what we want to be doing right now. We do not want to see any Russian military incursion into Ukraine.”

Ukrainian President Volodymyr Zelenskyy on Friday alleged that his country’s intelligence service has uncovered plans for a Russia-backed coup d’etat in Ukraine next week, something which the Russian government denied.

Asked about the possible coup plot, Donfried said: “We are in touch with the Ukrainian government to discuss this further, and we’re working to obtain additional information.”

Samuel Petrequin in Brussels and AP National Security Writer Robert Burns in Washington contributed to this report.