WASHINGTON (REUTERS) - US President Donald Trump has employed tough rhetoric in response to North Korea's missile tests, but his options appear limited in dealing with a challenge that has vexed his Oval Office predecessors.

Secretary of State Rex Tillerson called Pyongyang's test launch of an intercontinental ballistic missile (ICBM) on the eve of US Independence Day on Tuesday (July 4) a global threat, as experts cautioned that Alaska may now be within range.

Most options for cracking down on Pyongyang fall into four categories: economic sanctions, covert action, diplomatic negotiations and military force.

ECONOMIC SANCTIONS

North Korea is already among the most heavily sanctioned nations, facing numerous strictures to limit its ability to conduct commerce, participate in international finance and trade in weapons and other contraband.

Despite those measures, "most analysts agree that US and multilateral sanctions have not prevented North Korea from advancing its fledgling nuclear weapons capability", said a report last year from the US Congressional Research Service.

Reuters has reported that Mr Trump is focusing his North Korea strategy for now on tougher sanctions, possibly including an oil embargo, banning its airline, intercepting cargo ships and punishing Chinese banks doing business with Pyongyang.

US officials have expressed doubt about how much further China is willing to go to pressure its defiant North Korean ally - despite Beijing's increasing frustration with Pyongyang's missile launches and nuclear tests.

Beijing has long feared that economic collapse in North Korea would flood China with refugees and leave it to deal with chaos on the Korean peninsula.

COVERT ACTION

The US, with help from Israel, temporarily set back Iran's nuclear programme via a computer virus called Stuxnet, which destroyed thousands of centrifuges used to enrich uranium.

The US tried, but failed, to deploy a version of the Stuxnet virus to attack North Korea's nuclear weapons programme in 2009 and 2010, Reuters reported in 2015.

One former high-ranking intelligence official briefed on the programme said the effort was stymied by North Korea's utter secrecy and extreme isolation of its communications systems.

That same secrecy is responsible for what US officials have consistently described as limited US intelligence about the North Korean government's inner workings.

Another semi-covert approach would be for Washington to use electronic warfare or cyber attacks to disable North Korean missiles during or shortly after their launch.

The high failure rate of the North's missile tests has prompted speculation that the US is already doing so. The New York Times reported earlier this year that the US military was working hard on missile defences that would involve sabotage rather than traditional anti-missile interceptors.

DIPLOMACY

The Trump administration has said it is open to diplomatic negotiations with North Korea, but only under the right conditions. The talks need to be aimed at denuclearisation, Washington has said.

There have been no official negotiations for seven years. In February 2012, the US and North Korea announced an agreement in which the North would suspend operations of its Yongbyon uranium enrichment plant, allow international inspectors to verify the suspension, and implement moratoriums on nuclear and long-range missile tests.

In return, North Korea would get badly needed food aid.

In April of that year, the North attempted to launch a satellite on a three-stage rocket, in what Washington said was a violation of the agreement because of the rocket's potential military uses. While Pyongyang denied it had breached the agreement, the deal was suspended.

While China has responded to previous North Korean tests of suspected ICBM technology by agreeing to tougher UN sanctions, it emphasised on Tuesday its call for a return to talks with North Korea by signing a joint statement with Russia.

Under a plan devised by Beijing, North Korea would suspend its ballistic missile programme in return for a moratorium on large-scale military exercises by the US and South Korea, which Washington and Seoul say are essential to maintain defence readiness.

MILITARY FORCE

Military options available to Mr Trump range from a sea blockade aimed at enforcing sanctions to cruise missile strikes on nuclear and missile facilities to a broader campaign aimed at overthrowing leader Kim Jong Un.

North Korea has threatened to "ruthlessly ravage" the US if Washington chooses to attack.

US Defence Secretary Jim Mattis has warned the consequences of any military action would be "tragic on an unbelievable scale". Any US military action brings severe risks for US ally South Korea.

On the same programme, Mr Trump's national security adviser H.R. McMaster has indicated military force is a last resort. "It's time for us to undertake all actions we can, short of a military option, to try to resolve this peacefully," he said.