America's top diplomat heads to Moscow this morning with key questions over Syria unanswered and growing tensions brewing on the Korean Peninsula.

US allies are moving ahead with their own policies for dealing with Syria after attempts by top Trump Administration officials to articulate a plan left elements unresolved in the wake of missile strikes on President Bashar al-Assad's military.

And a US Navy strike group led by a nuclear-powered aircraft carrier is en route to the western Pacific with talk of military action by the US gaining traction following the Syria strikes.

Left unclear was whether Assad must relinquish power, how displaced Syrians would be protected and when the US might feel compelled to take further action. US President Donald Trump ordered the strikes last week following Assad's use of chemical weapons.

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After meeting US Secretary of State Rex Tillerson, British Foreign Secretary Boris Johnson floated the possibility of new sanctions on both the Syrian and Russian militaries, an idea the US has only briefly mentioned. Johnson also said the US could launch more cruise missiles into Syria.

Tillerson himself raised fresh expectations for aggressive US action - and not only in Syria - as he visited Sant'Anna di Stazzema, a Tuscan village where the Nazis massacred more than 500 civilians during World War II. "We rededicate ourselves to holding to account any and all who commit crimes against the innocents anywhere in the world," Tillerson said.

Though such comments hint at a more activist US foreign policy focused on preventing humanitarian atrocities, Trump has consistently suggested he prefers the opposite approach. His new Administration has generally downplayed human rights concerns while promoting an "America First" strategy de-emphasising the concerns of foreign nations. The uncertain view of US objectives prevailed as Tillerson planned to attend a meeting today of countries that share a similar approach to resolving Syria's protracted civil war. The session was taking place on the sidelines of the Group of 7 summit in Italy.

Tillerson was then scheduled to fly to Moscow, the first official visit by a Trump Cabinet official to Russia, Assad's strongest ally. The US has said its Syria strategy centres on persuading President Vladimir Putin to stop supporting Assad. Yesterday, the US raised the stakes significantly by accusing Russia of knowing in advance of the chemical attack and using a Russian-operated drone to help cover it up.

In Asia, South Korea's acting leader has warned of "greater provocations" by North Korea as tension on the Korean Peninsula rises over concern the North may conduct a test of its military hardware in coming days.

South Korean acting President Hwang Kyo Ahn ordered the military to intensify monitoring of the North's activities and to ensure close communication with the US. "It is possible the North may wage greater provocations such as a nuclear test timed with various anniversaries including the Supreme People's Assembly," said Hwang.

The North has convened a Supreme People's Assembly session, one of its twice-yearly sessions in which major appointments are announced and national policy goals are formally approved.

The North's Foreign Ministry, in a statement carried by its KCNA news agency said that the US Navy strike group's move near the Korean Peninsula showed America's "reckless moves for invading had reached a serious phase".

"We never beg for peace but we will take the toughest counteraction against the provocateurs in order to defend ourselves by powerful force of arms and keep to the road chosen by ourselves," an unidentified ministry spokesman said.

- AP, Reuters