The top U.S. military commander in the Pacific warned the situation in North Korea is a “recipe for disaster,” as the region prepares for Donald Trump’s first visit to the region as president.

Admiral Harry Harris, head of the U.S. Pacific Command, said in a speech today in Singapore that China must do more to build pressure on North Korea over its weapons programs. The U.S. government will continue to be presented with military options for dealing with Kim Jong Un, he said, although he added a diplomatic solution remained the priority.

“Combining nuclear warheads with ballistic missiles in the hands of a volatile leader, Kim Jong Un, is a recipe for disaster,” Harris said. “Many people have thought about military options being unimaginable regarding North Korea. Folks, I must imagine the unimagined.”

Harris’s remarks at the International Institute for Strategic Studies event came amid increasing saber-rattling between the Trump administration and North Korea. There have been signs that Kim’s regime is preparing more missile tests as the U.S. and its ally, South Korea, conduct joint drills.

Trump’s effort to halt North Korea’s drive to build a nuclear weapon capable of reaching the continental U.S. is expected to be a topic of contention when the U.S. president makes his first swing through the region from Nov. 3 to Nov. 14. His stops include Japan, South Korea and China, which the U.S. has accused of enabling Kim’s administration with economic and political support.

“If you’re hoping for a diplomatic, economic solution, a peaceful solution to the North Korean crisis, then those roads go through Beijing in my mind,” Harris said during a question and answer session after the speech. “And I think China recognizes this now.”

The U.S.-South Korean marine exercises, including the supercarrier USS Ronald Reagan, started Monday and will continue through Friday on both sides of the Korean Peninsula. South Korean media reported over the weekend that North Korean “transporter erector launchers” had been observed carrying ballistic missiles near Pyongyang and North Pyongan province.

Tensions often rise around such drills, which North Korea views as rehearsals for invasion. The country’s ambassador to the United Nations, Kim In Ryong, warned on Monday that nuclear war “may break out any moment.”

While Harris put much of the onus on China, he also expressed uncertainty about another North Korean neighbor and key player in the international campaign to pressure Kim: Russia. Russian President Vladimir Putin has questioned the effectiveness of greater sanctions, even while his country votes for them on United Nations Security Council.

“They can be very helpful or they can be the opposite,” Harris said in response to a question, referring to Russia. “It remains to be seen where Russia is completely. But I think that Russia can be a spoiler here, if it wants to.”

North Korea conducted its sixth and most powerful nuclear test on Sept. 3, and has launched more than a dozen rockets this year, including two intercontinental ballistic missiles that could reach as far as the U.S. East Coast. The country views the weapons as a way to deter an eventual U.S. attack that could topple the regime, such as in Iraq and Libya.

That scenario also worries China, which is anxious that North Korea’s collapse could bring a unified — and U.S.-allied — Korea on its border. Harris said that an audience member’s proposal to assuage China by offering to extend the demilitarized zone to the Chinese frontier was “a good one.”

“I’m not the guy that’s going to be there to advocate for it, but I think all these ideas in this space ought to be encouraged,” Harris said. “China fears a unified Korean peninsula that’s friendly to the United States and U.S. troops on its border. I think China has come to the realization that a nuclear exchange or some kind of war is going to be even worse.”