Despite North Korea’s failed missile test Sunday, the regime’s nuclear program has “accelerated in recent months,” a former U.S. ambassador to South Korea warns.

“In 2016 they had two nuclear tests. They’re kind of ready to go for another one any day now. They’ve also had something on the order of 25 or 30 missile tests, so they really accelerated their program, and I tell you, there’s no amount of patience that’s going to dissuade that,” said Christopher Hill, now dean of the Josef Korbel School of International Studies at the University of Denver.

“They’re really going full throttle for a nuclear option that could include holding the U.S. at risk,” he added. “Very serious business, and really it’s something that we have to deal with as a sort of top-echelon issue.”

Amid rising tensions, Vice President Mike Pence, who’s visiting South Korea, said Monday the “era of strategic patience is over” when it comes to dealing with the North. Standing with South Korea’s acting president, Hwang Kyo-ahn, Pence said “all options are on the table” to deal with threats from the regime and its third-generation dictator Kim Jong Un – or “Kim 3.0,” according to Hill.

Hill, who served as ambassador under President George W. Bush and went on to become assistant secretary of state for East Asian and Pacific affairs, said Pence’s trip to the Republic of Korea is “very important.”

“What needs to be done now is a real full-court press with the Chinese, and what’s very, very important is to work closely with the South Koreans because every option may be on the table, but we don’t have the option of doing something without consulting with them,” Hill said.

The North is also not showing any signs of backing off its nuclear program, Hill said.

“Kim Jong Un has shown no interest in negotiation and he’s not interested in what we have to say and, frankly, what the Chinese have to say. We’ve got a tall order in the coming months and perhaps years,” Hill said.

While Pence reiterated Monday that President Trump was hopeful China will use “extraordinary levers” to pressure the North to abandon its nuclear weapons, Hill pointed to questions surrounding the sincerity of China’s recent commitments.

In line with U.N. Security Council sanctions, China suspended all coal imports from North Korea in February. But according to the Washington Post, Chinese customs data showed “imports and exports between the two countries were up sharply in the first quarter” of 2017. Hill said there are theories that the reason China doesn’t buy as much North Korean coal is because their industrial sector doesn’t need it.

“The truth of the matter is the Chinese government, powerful as it is, is not in a position to really throttle down on every Chinese enterprise, especially at the northeast China. … And when the Chinese can’t do something, they often make virtue of their inability to control things by calling on the Americans to do more negotiations with the North Koreans, etc. when they themselves haven’t been negotiating,” Hill said.

“So it’s a work in progress with China. The U.S. has to kind of convince the Chinese we’re not looking for strategic advantage over them. We’re not going to gloat about anything we do vis-à-vis the Chinese. But we do need China to kind of step it up and step up, really, in dealing with North Korea because this problem is dangerous. It’s dangerous to our interests and frankly dangerous our alliances with South Korea and Japan. And I think in the fullness of time, China should realize it’s not a bad thing for the U.S. to have alliances with these two countries.”