Tensions between President Donald Trump and Secretary of State Rex Tillerson, right, have come to a head on several major issues. Evan Vucci/AP Republican Sen. John McCain and Democratic Sen. Ben Cardin sent a letter to President Donald Trump late last month expressing concern that his administration had not yet implemented new sanctions against Russia that he signed into law on August 2.

"As critical deadlines are approaching, it is imperative that your Administration implement the law to its fullest extent to uphold and protect American interests," the senators wrote in the letter, which they made public on Wednesday.

They added: "Given the ongoing threat that Russia poses to the US and our other allies, we are particularly concerned about the need for vigorous enforcement of the sanctions against Russia."

Trump, who has expressed lingering doubts that Russia interfered in the 2016 US presidential election, said shortly after signing the bill into law that "America will not tolerate interference in our democratic process," and he denounced "Russian subversion and destabilization."

But the president appears to have blown past the October 1 deadline McCain and Cardin gave the White House to clarify which entities the administration planned to sanction within Russia's defense and intelligence sectors.

McCain is the chairman of the Senate Armed Services Committee, and Cardin is the ranking member of the Senate Foreign Relations Committee. The pair had requested a briefing from the State Department and the Treasury on the administration's "overall sanctions implementation plan with respect to Russia," their letter said, but never received it.

"The delay calls into question the Trump administration's commitment to the sanctions bill which was signed into law more than two months ago, following months of public debate and negotiations in Congress," the senators said in a joint statement on Wednesday. "They've had plenty of time to get their act together."

The White House did not respond to a request for comment.

Trump has called the sanctions legislation, which also targets Iran and North Korea, "seriously flawed." He said shortly after signing the bill that it "improperly encroaches on executive power, disadvantages American companies, and hurts the interests of our European allies." He tweeted later that Congress was to blame for the US's relationship with Russia being at an "all-time & very dangerous low."

A provision in the sanctions law requiring Trump to get congressional approval before altering or lifting sanctions on Russia was a major point of contention between the White House and Congress. But Trump was all but forced to sign the legislation, which had a veto-proof majority.

"As you know, the law provides for Congress to review any administration determination to remove sanctions designations on individuals or entities," McCain and Cardin wrote in their September 28 letter to Trump. "Based on the overwhelming Congressional support for enacting this law, and that provision in particular, Congress will undoubtedly take that role seriously."

Read McCain and Cardin's full letter: