US Secretary of State Mike Pompeo visited American troops in Saudi Arabia Thursday after talks with King Salman on the second day of a visit focused on countering Iran.

The United States began building up its military presence at the Prince Sultan air base, south of Riyadh, last year following a series of attacks in the Gulf that Washington and Riyadh have blamed on their common foe Iran.

"Pompeo's visit to Prince Sultan air base and a nearby US Patriot battery highlights the long-standing US-Saudi security relationship and reaffirms America's determination to stand with Saudi Arabia in the face of Iranian malign behaviour," the State Department said in a statement.

It said the US deployment of missile defence systems and fighter jets was part of "a defensive mission to deter and protect against any future attacks".

Pompeo's three-day visit to close US ally Saudi Arabia comes in the wake of a US-ordered drone strike that killed Qasem Soleimani, Iran's most powerful general, in Baghdad on January 3.

The killing sparked a surge in regional tensions and Iran responded with missile strikes on bases hosting US forces in Iraq. US President Donald Trump and his oil-rich ally Riyadh have accused Iran of attacking tankers in the Gulf and Saudi oil installations, incidents which roiled global energy markets last year. Tehran denies involvement in the attacks and Riyadh has since appeared keen to engage in cautious diplomacy to ease friction.