A Russian spy ship was spotted patrolling off the coast of Delaware on Tuesday — the same day it emerged that the Kremlin secretly deployed new cruise missiles and buzzed a US Navy destroyer, according to a report.

The Russian ship SSV-175 Viktor Leonov was in international waters, 70 miles from Delaware, as it sailed north at 10 knots, a US official told Fox News. It was not immediately clear where it was headed.

Russia deployed ground-launched cruise missiles to two locations inside the country in December, a US official told Fox News later Tuesday.

The New York Times reported that the Obama administration had previously seen the missiles as a violation of a 1987 arms control treaty between the US and Russia that banned ground-launched intermediate-range missiles.

Russia had secretly deployed the SSC-8 cruise missile that Moscow has been developing and testing for several years, the Times reported.

The missile deployment presents a major challenge for President Trump, who has vowed to improve relations with Russian strongman Vladimir Putin and to pursue future arms accords.

The Russian action also comes as the Trump administration is struggling to fill major policy positions at the State Department and the Pentagon — and to find a permanent replacement for Michael Flynn, the national security adviser who resigned Monday.

He stepped down after it was revealed that he had misled Vice President Mike Pence and other US officials over conversations with Moscow’s ambassador to Washington.

Adding to the saber-rattling, Fox News confirmed a report from the Washington Free Beacon that four Russian jets buzzed the USS Porter on Friday.

“There were several incidents involving multiple Russian aircraft,” Navy Capt. Danny Hernandez, spokesman for the European Command, told the Washington Free Beacon. “They were assessed by the commanding officer as unsafe and unprofessional.”

The Russian ship, which last sailed near the US in April 2015, is capable of intercepting communications and also measuring US Navy sonar capabilities, officials told Fox News. It is also armed with surface-to-air missiles.

“It’s not a huge concern, but we are keeping our eyes on it,” one official said.

There are four US Navy warships in the Atlantic off the coast of Norfolk participating in normal training, but none has been deployed to shadow the Russian spy ship.

This Russian missile action follows recent test launches by Iran and North Korea.

The State Department concluded in a July 2014 arms control report that “the Russian Federation is in violation of its obligations under the INF Treaty not to possess, produce, or flight-test a ground-launched cruise missile (GLCM) with a range capability of 500 km to 5,500 km (310 miles to 3,420 miles), or

to possess or produce launchers of such missiles.”

Russia accused Washington of conducting “megaphone diplomacy” after the accusation was repeated by the State Department in 2015. Moscow also denied it had violated the Intermediate-Range Nuclear Forces treaty, which helped end the Cold War between the two countries.

The New York Times said the Obama administration had tried to persuade Moscow to correct the violation while the missile was still in the testing phase.

Russia now has two battalions of the cruise missile, the newspaper quoted administration officials as saying. One is located at Russia’s missile test site at Kapustin Yar in the country’s southeast.

The other battalion has been located at an operational base elsewhere in Russia, the Times quoted one unidentified official as saying.

With Post wires