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President Donald Trump and Germany's Angela Merkel are “concerned” by Russia’s boasts about nuclear weapons, an official said Friday after the leaders spoke by phone.

Their discussion came after Vladimir Putin said in his annual state of the nation address that Russia had developed new nuclear-capable weapons, including a nuclear-powered intercontinental ballistic missile that renders Western defense systems "useless.”

The Russian president also warned that any use of nuclear weapons against its allies would be regarded as a nuclear attack against Moscow and would be met with a response. “Nobody should have any doubt about that,” he said.

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Trump and Merkel, the German chancellor, spoke by phone late Thursday, German government spokesman Steffen Seibert said in a statement.

The two leaders are “concerned about Russian President Putin's recent remarks on arms development and its negative impact on international arms control efforts,” he said.

In an exclusive interview with NBC News Thursday, Putin denied the new missiles amounted to a new Cold War. He said the new weapons were Moscow's response to the United States' withdrawal in 2002 from the 1972 Anti-Ballistic Missile Treaty, which prohibited nationwide missile defenses.

"If we are to speak of an arms race, then an arms race started precisely at that point," Putin told Megyn Kelly in Moscow.

Trump and Merkel also spoke about Syria, and agreed that a U.N. Security Council resolution calling for a 30-day cease-fire in Syria must be implemented immediately, Seibert said.

France said Trump had also called President Emmanuel Macron about the issue.

Macron promised “a firm response would take place” in coordination with Washington “if there is proven use of chemicals resulting in the death of civilians," the Elysee Palace said in a statement.

Alastair Jamieson reported from London, and Nancy Ing from Paris.