Nearly half of the Senate Republican conference is pressing President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE to adopt a tough stance on nuclear arms control with Russian President Vladimir Putin in the wake of Russian aggression against Ukrainian naval forces on the Sea of Azov.

Trump announced Thursday that he would cancel a scheduled meeting with Putin at the Group of 20 (G-20) summit in Buenos Aires, Argentina, after Russian forces captured three Ukrainian naval ships during a territorial dispute.

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“Based on the fact that the ships and sailors have not been returned to Ukraine from Russia, I have decided it would be best for all parties concerned to cancel my previously scheduled meeting in Argentina,” Trump said in a series of tweets.

The two leaders were expected to discuss nuclear arms control on the sidelines of the summit.

Putin has expressed interest in extending the Obama-era New START nuclear treaty, which Congress ratified in 2010, but Trump has panned as a bad deal for the United States.

The treaty expires in 2021.

Earlier on Thursday, 25 Republican senators sent a letter to Trump urging him to insist on modernization of the U.S. arsenal and better compliance by Russia with the arms control accord.

“The value of the Treaty depends on a sustained and vigorous U.S. nuclear weapons modernization program, strict compliance by Russia with its arms control obligations, and a true balance of nuclear capabilities between the parties to the Treaty,” they wrote.

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The senators wrote that U.S. maintenance and modernization of its nuclear weapons stockpile has fallen behind what was promised to Congress when it ratified New START in a lame-duck session eight year ago.

They argue that continued funding for modernization programs such as the development of low-yield warhead options are necessary “in the face of dangerous international security developments since the New START was ratified.”

The lawmakers assert that Russia is “in material breach of its arms control commitments” such as the Intermediate-Range Nuclear Forces Treaty because of its deployment of ground-launched cruise missiles.

They also faulted Russia for not addressing the disparity in tactical nuclear weapons stockpiles between the two countries and instead increasing “the role of nuclear forces and their types and variety since 2010.”

Trump last month threatened to pull out of Intermediate-Range Nuclear Forces Treaty after charging that Russia “violated the agreement.”

“We’ll have to develop those weapons,” he told reporters at a campaign event in Nevada. “We’re going to terminate the agreement and we’re going to pull out.”

Trump’s tough stance on arms-control agreements with Russia has strong support in the Senate GOP conference.

“We know you agree that arms control is not an end to itself; it is but a single tool that may be used to advance U.S. national security when carefully considered,” the 25 senators wrote. “We look forward to continuing to work with you throughout the review process.”