Belarusian President Alexander Lukashenko, a close ally of Russia’s Vladimir Putin, has allowed Moscow to use Belarusian territory as a staging ground for the invasion of Ukraine, but has not directly taken part in the conflict.

That could change as soon as Monday, a senior U.S. intelligence official with direct knowledge of current U.S. intelligence assessments told The Associated Press. The official said whether Belarus enters the war depends on the talks between Russia and Ukraine.

Ukrainian President Volodymyr Zelenskyy has sent a delegation from his country to meet their Russian counterparts at an unspecified time and location on the Belarusian border on Monday, as Russia’s military assault on Ukraine entered its fifth day.

He earlier rejected Belarus as a location for the diplomatic talks, saying Belarus was complicit in the Russian assault on Ukraine.

Zelenskyy said Lukashenko has reassured him that he would not join Moscow’s war effort during a phone call on Sunday, according to The Kyiv Independent.

But sources believe Lukashenko will have no choice but to join Russia. “It’s very clear Minsk is now an extension of the Kremlin,” a U.S. administration official told The Washington Post.

On Sunday, Anton Herashchenko, an adviser to Ukraine’s interior minister, said Russia had used Iskander missile systems to attack Zhytomyr International Airport. The airstrikes were conducted from Belarus using Russian ballistic missile launchers, Herashchenko said in a statement on Facebook.

Tensions escalated further on Sunday when Russian President Vladimir Putin ordered the nation’s nuclear deterrent forces to be put on high alert, raising fears that the invasion of Ukraine could lead to nuclear war. Putin cited “aggressive statements” made by NATO countries toward Russia and tough economic sanctions.

White House Press Secretary Jen Psaki said Putin’s move was part of a “pattern” of “manufacturing threats that don’t exist.”

“This is really a pattern that we’ve seen from President Putin through the course of this conflict, which is manufacturing threats that don’t exist in order to justify further aggression,” Psaki said on ABC’s This Week. “And the global community and the American people should look at it through that prism.”

Meanwhile, it was reported on Sunday that Belarus will allow Russia to place nuclear missiles in the country following a referendum, in a reversal of policy that could see nuclear weapons on Belarusian territory for the first time since the country gave them up after the fall of the Soviet Union.

Lukashenko said he could ask Russia to return nuclear weapons to Belarus, Reuters reported.

“If you (the West) transfer nuclear weapons to Poland or Lithuania, to our borders, then I will turn to Putin to return the nuclear weapons that I gave away without any conditions,” he said.