U.S. Sen. Ed Markey, D-Massachusetts, called for immediate direct diplomacy and tougher sanctions against North Korea Tuesday, following reports that Pyongyang has successfully produced a miniaturized nuclear warhead.

Markey, who serves as the top Democrat on the Senate Foreign Relations Committee's East Asia sub-panel, renewed his call for the Trump administration to take a diplomatic approach to addressing North Korea's nuclear threat.

Contending that "a miniature nuclear warhead in North Korea is a massive and unacceptable threat to the United States," the Massachusetts Democrat argued that President Donald Trump needs to adopt a "coherent and comprehensive strategy" -- not one of "bluster and saber rattling."

That strategy, he argued, must focus on "immediate direct diplomacy and increased economic pressure, along with military deterrence with regional allies to confront this grave national security threat."

Although the United Nations Security Council recently imposed new sanctions to slash North Korean exports by upwards of $1 billion, the senator contended, the Trump administration "should not pat itself of the back and assume that it has done its job.

"For these sanctions to be effective, the Trump administration must immediately work to ensure all countries quickly and fully implement them and must go even farther to pressure China, Russia, and North Korea's other enablers," he said in a statement.

Markey further urged Trump to immediately begin direct talks with North Korea, cautioning that "the consequences of inaction are too great."

The Massachusetts Democrat's call for stricter sanctions and diplomacy came shortly after the Washington Post reported that U.S. intelligence officials believe North Korea was successful in producing the miniaturized nuclear warhead that could be placed in an intercontinental ballistic missile capable of striking the United States -- an important mark in the country's effort to become a nuclear power.

U.S. officials, in a new confidential analysis, calculated that North Korean Leader Kim Jong Un now controls up to 60 nuclear weapons, the Post reported.

Trump, meanwhile, warned North Korea Tuesday against making "any more threats to the United States," saying they "will be met with fire and fury like the world has never seen," according to the Associated Press.