The US' "strategic patience" with nuclear-armed North Korea is over, Tillerson said in Seoul on Friday.

This signals a clean break from the position of the previous administration, when the US ruled out engaging the North until it made a tangible commitment to de-nuclearization.

"The policy of strategic patience has ended," Tillerson said at a joint press conference with his South Korean counterpart Yun Byung-Se. "We are exploring a new range of diplomatic, security, economic measures. All options are on the table."

Tillerson added that it may be necessary to take pre-emptive military action against North Korea if the threat from its weapons program reaches a level "that we believe requires action." He didn't say what that level could be.

"Certainly we do not want to, for things to get to military conflict," he told reporters.

Tillerson arrived at the heavily fortified border with North Korea, the Demilitarized Zone (DMZ), on Friday. The US Secretary of State said that 20 years of diplomatic efforts in relation to North Korea had "come to nothing."

He noted a period when the US provided North Korea with $1.35 billion (1.28 billion euros) in assistance "to take a different pathway," adding that it had not worked.

Tillerson and Japanese Prime Minister Shinzo Abe agreed this week that their two countries should share strategic goals to deal with Pyongyang's growing nuclear missile threat.

Watch video 00:38 China urges N.Korea to stop missile tests

Threats from the North

North Korea has a long-standing ambition to become a nuclear power and conducted its first underground atomic test in 2006, in the face of global opposition.

Four more tests have followed, two of them last year.

Pyongyang has continued to defy the international community, even after two rounds of UN-backed sanctions, and last week test-fired a salvo of missiles that fell in waters off Japan.

South Korea, meanwhile, has agreed to deploy a US Terminal High Altitude Area Defense (THAAD) missile system in South Korea.

After visiting the DMZ, Tillerson is due to meet Foreign Minister Yun Byung-se and Prime Minister Hwang Kyo-ahn, who is also acting president.

Tillerson began his first Asian visit as secretary of state in Japan on Wednesday and travels to China on Saturday.

'Beijing could do more'

Washington has been pushing Beijing to do more to help reduce North Korea's nuclear and missile programs.

Tillerson is expected to tell the Chinese leadership that the US intends to increase missile defenses in the region, despite China's opposition, a US official told the news agency Reuters in Washington.

He will also press the North's key diplomatic protector and trade partner to back tougher sanctions. Beijing meanwhile has been angered by the deployment of the THAAD missile defence system in the South.

THAAD on the table?

The liberal South Korean politician most likely to become the country's next president said on Friday he would if elected review THAAD and would consult China.

If Moon Jae-in, the front-runner for the May 9 presidential election, reverses policy on the deployment of THAAD, it would place him at odds with the US, South Korea's biggest ally.

The conservative government of impeached president Park Geun-hye agreed to deploy THAAD to guard against an attack by North Korea.

The South China Seas dispute

During his Senate confirmation hearing for secretary of state, Tillerson compared China's island-building and deployment of military assets to Russia's 2014 annexation of Crimea and suggested China's access to the islands should not be allowed.

The topic is likely to be high on the agenda when Tillerson visits Beijing for talks with top officials on Saturday and Sunday.

China, the Philippines, Vietnam, Taiwan, Malaysia and Brunei contest ownership of the South China Sea, which straddles one of the world's busiest sea lanes and is believed to sit atop vast deposits of oil and gas.

jbh/rt (Reuters, AP, AFP)