Sen. John McCain of Arizona. AP Photo/Jacquelyn Martin Sen. John McCain, R-Ariz., on Thursday criticized President Trump for "playing right into the hands" of Russian President Vladimir Putin following a report that the president ended a covert program aiding Syrian rebel groups.

"If these reports are true, the administration is playing right into the hands of Vladimir Putin," McCain, chairman of the Senate Armed Services Committee, said in a statement Thursday. "Making any concessions to Russia, absent a broader strategy for Syria, is irresponsible and short-sighted."

McCain's statement came hours after news broke that the 80-year-old has been diagnosed with brain cancer, and was a sign that the Arizona senator plans on making his opinions known while he's recuperating after surgery.

The president ended the CIA program to provide arms and supplies to Syrian rebel groups fighting Syrian President Bashar al-Assad, according to a Washington Post reportWednesday

Officials said Trump made his decision more than a month ago. The end of the program to aid Syrian rebels, authorized by former President Barack Obama, was never publicly announced.

Though controversial, US support for rebels had bipartisan support. Obama began shipping arms and supplies to Syrian rebels in 2013, and Congress eased restrictions on the program last year.

Russian President Vladimir Putin and US President Donald Trump. AP

Trump's decision is likely to be well-received by the Russians, who backed Assad and attacked rebel groups aided by the US, and accused the US of backing terrorist groups.

In addition to criticizing Trump's decision to end the program arming Syrian rebels, McCain also slammed the Trump administration for failing to outline a strategy for Syria and the Middle East.

"The administration has yet to articulate its vision for Syria beyond the defeat of ISIL, let alone a comprehensive approach to the Middle East," he said. "A key pillar of American strategy must be the removal of Assad from power as part of an end to the brutal conflict in Syria, which has fueled ISIL's growth through its cruelty, extended malign Iranian influence, and undermined broader regional stability."

"Six months into this administration, there is still no new strategy for victory in Afghanistan either," he continued. "It is now mid-July, when the administration promised to deliver that strategy to Congress, and we are still waiting."

The senator remains in Arizona after undergoing surgery to remove a blood clot from above his left eye. Tissue extracted during the surgery proved to be cancerous.

The senator's office has not provided a timetable for when he will return to Washington, D.C., though McCain and his family are reviewing treatment options.