Image copyright AFP Image caption North Korea has been releasing photos of mass live-fire exercises

The US is to tighten sanctions on North Korea and step up diplomatic moves aimed at pressuring the country to end its nuclear and missile programmes.

President Donald Trump's strategy was announced after a special briefing for all 100 US senators.

Earlier, the top US commander in the Pacific defended the deployment of an advanced missile defence system in South Korea.

Tensions have risen amid fears the North is planning new weapons tests.

"The United States seeks stability and the peaceful denuclearisation of the Korean peninsula," said a joint statement issued by Secretary of State Rex Tillerson, Defense Secretary Jim Mattis and Director of National Intelligence Dan Coats.

Image copyright EPA Image caption All 100 senators including Chris Coons (centre) from Delaware were briefed by the Trump administration on developments in North Korea

Image copyright EPA Image caption South Korean acting President Hwang Kyo-ahn (centre, front) inspected a variety of firearms during a firing drill north of Seoul on Wednesday

"We remain open to negotiations towards that goal. However, we remain prepared to defend ourselves and our allies.

"The president's approach aims to pressure North Korea into dismantling its nuclear, ballistic missile, and proliferation programs by tightening economic sanctions and pursuing diplomatic measures with our allies and regional partners," the statement added.

The US already has extensive sanctions in place on North Korea, including a blanket ban on trade and a blacklist of anyone dealing with North Korea.

It is not clear what further sanctions Washington could impose.

The BBC's Barbara Plett Usher in Washington

The Americans are deeply concerned about advances in North Korea's weapons technology, they believe it could well be capable of hitting the United States with a nuclear warhead before the end of President Trump's first term.

Any US military intervention to pre-empt that would be fraught with risk, but Mr Trump has toughened his rhetoric to drive home a message that it's a credible threat.

A key part of his plan is to pressure China to lean more heavily on North Korea to dismantle its nuclear weapons programme. The statement says the US is open to negotiations towards that end, but many even in Washington doubt the regime could ever accept such terms.

Democratic Senator Christopher Coons told reporters that military options were discussed at the special presidential briefing for senators.

"It was a sobering briefing in which it was clear just how much thought and planning was going into preparing military options if called for - and a diplomatic strategy that strikes me as clear-eyed and well-proportioned to the threat," he said.

Image copyright AFP Image caption North Korea released a photo of Mr Kim saluting troops

A White House official said an option under consideration was to put North Korea back on the state department's list of countries that sponsor terrorism.

Mr Trump's predecessor, Barack Obama, imposed sanctions over a year ago following a nuclear test and satellite launch by the North.

North Korean government property in America was frozen and US exports to, or investment in, North Korea was banned.

The order also greatly expanded powers to blacklist anyone, including non-Americans, dealing with North Korea.

The senators received a highly unusual briefing by the Trump administration on the seriousness of the threat from North Korea and the president's strategy for dealing with it on Wednesday.

Image copyright YONHAP Image caption Locals protested as six trailers carrying Thaad missile equipment were transported to Seongju in South Korea

Earlier Admiral Harry Harris, head of US Pacific Command, said the US would be ready "with the best technology" to defeat any missile threat.

The deployment of Terminal High-Altitude Area Defense (Thaad) system in South Korea was aimed, he argued, at bringing North Korean leader Kim Jong-un "to his senses, not to his knees".

Adm Harris said he believed that North Korea would try to attack the US as soon as it had the military capabilities.

China says the deployment of Thaad will destabilise security and there have been protests in South Korea itself, where three people were injured in clashes with police as the system was being delivered to a former golf course on Wednesday.