U.S. Secretary of State John Kerry is meeting China's top leaders in Beijing to press them to exert more pressure on ally North Korea.
Speaking to China's President Xi Jinping on April 13, Kerry said, "This is obviously a critical time with some very challenging issues."
He cited tensions in the Korean Peninsula, but also Iran's nuclear program and the conflict in Syria.
Pyongyang has issued weeks of threats of an impending war following the imposition of UN sanctions after North Korea's third nuclear test in February.
In Seoul on April 12, Kerry said the United States would never accept North Korea as a nuclear power.
Kerry also said it would be a "huge mistake" if North Korea were to go ahead with an anticipated medium-range missile test.
"I would say, ahead of time, that it is a huge mistake for [North Korean leader Kim Jong Un] to do that because it will further isolate his country and further isolate his people who, frankly, are desperate for food, not missile launches."
South Korea and Japan have been on high alert amid indications that Pyongyang was preparing for a rocket test at any time after it moved two Musudan ballistic missiles to its east coast.
Kerry also repeated the United States will do what it must to defend itself and its allies.
"The rhetoric that we're hearing from North Korea is simply unacceptable by any standard. And I am here to make it clear today on behalf of President [Barack] Obama, and the citizens of the United States, and our bilateral security agreement that the United States will, if needed, defend our allies and defend ourselves."
Kerry urged China -- Pyongyang's only major ally -- to "put some teeth" into efforts to press Pyongyang to abandon its nuclear ambitions.
North Korea refuses to give up its nuclear weapons that it called on April 12 its "treasured" guarantor of security.
South Korean President Park Geun-hye on April 12 said Seoul was also open to holding talks with Pyongyang to calm the tensions.
Meanwhile, the White House said on April 12 that North Korea had not demonstrated the capability to deploy a nuclear-armed missile, a day after the Pentagon's intelligence agency said that the North had a nuclear missile capacity.
With reporting by AP and Reuters