Yemeni government forces pressed further into the strategic port city of Hodeidah, seizing its main hospital in heavy fighting Saturday, as their Saudi-led coalition backers put a brave face on an end to US refuelling support.

The United States said Tuesday it was preparing more sanctions against Moscow over the attempted assassination in Britain of a former spy, after a previous round sowed chaos on Russian markets.

Iran's President Hassan Rouhani said the Islamic republic "will proudly bypass sanctions" by the United States that took effect on Monday targeting the country's oil and financial sectors.

South Korea and the United States on Monday resumed small-scale military training that was indefinitely suspended following an historic summit between US President Donald Trump and North Korean leader Kim Jong Un.

US fighter jets darted over the Western Pacific on Saturday as the nuclear powered USS Ronald Reagan aircraft carrier joined Japanese destroyers and a Canadian warship for the biggest combat readiness war game ever staged in and around Japan.

President Donald Trump said he will seek to limit the right of citizenship for certain children born in the United States in a new bid to dramatically reshape immigration policies that was likely to spark a congressional battle.

US Defense Secretary Jim Mattis said on Sunday that he had met Saudi Arabia's foreign minister and called for a transparent investigation into the killing of journalist Jamal Khashoggi.

The United States sent two warships through the Taiwan Strait on Monday in the second such operation this year, as the US military increases the frequency of transits through the strategic waterway despite opposition from China.

Thousands of Honduran migrants whose trek toward the United States has triggered tirades from US President Donald Trump forged on Sunday after crossing a river into Mexico, marching for hours.

Mexican authorities allowed dozens of women and children from a US-bound Honduran migrant caravan to enter the country Saturday but thousands remain stranded on a border bridge between Guatemala and Mexico where riot police barred their progress.

Hundreds of people in a caravan of migrants that crossed from Honduras into Guatemala staged a dramatic bid to breach the Mexican border on Friday, as local governments began preparing to disperse the convoy under pressure from Washington.

So grave is the fallout from the disappearance of Saudi journalist Jamal Khashoggi that King Salman has felt compelled to intervene, five sources with links to the Saudi royal family said.