Masses of children skipped school Friday to join a global strike against climate change that teen activist Greta Thunberg said was "only the beginning" in the fight against environmental disaster.

Some four million people filled city streets around the world, organizers said, in what was billed as the biggest ever protest against the threat posed to the planet by rising temperatures.

Youngsters and adults alike chanted slogans and waved placards in demonstrations that started in Asia and the Pacific, spread across Africa, Europe and Latin America, before culminating in the United States where Thunberg rallied.

The United States announced Friday that it was sending military reinforcements to the Gulf region following attacks on Saudi oil facilities that it attributes to Iran, just hours after President Donald Trump ordered new sanctions on Tehran.

Trump said the sanctions were the toughest-ever against another country, but indicated he did not plan a military strike, calling restraint a sign of strength.

The Treasury Department renewed action against Iran's central bank after US officials said Tehran carried out weekend attacks on rival Saudi Arabia's oil infrastructure, which triggered a spike in global crude prices.