US Defense Secretary Mark Esper is welcomed by Saudi Arabia’s Deputy Defense Minister Prince Khalid bin Salman, in Riyadh, Saudi Arabia, October 21, 2019. Photo: REUTERS/Idrees Ali.

US Defense Secretary Mark Esper arrived in Saudi Arabia on Monday, with tensions simmering between the United States and Iran, and Russia seeking to increase its regional influence.

Esper is likely to meet King Salman and Crown Prince Mohammed bin Salman on his first trip to the key Middle East ally since he took office this summer, a visit intended partly to reassure Riyadh over bilateral ties.

US-Iran tensions have risen to new highs since May 2018, when the Trump administration withdrew from a 2015 international nuclear accord with Tehran that put limits on its nuclear program in exchange for the easing of sanctions.

The United States has deployed military forces to Saudi Arabia to bolster the kingdom’s defenses after an attack on oil sites last month.

Despite the additional troops, there are questions about the US commitment to allies in the region after Trump announced a sudden withdrawal from northeastern Syria, opening the door for Russia to increase its influence in the Middle East.

A senior US defense official, speaking on condition of anonymity, said the United States still wanted to be seen as the partner of choice in the region and Russia was not as dependable, whether it be the level of training or the military equipment it can provide.

President Vladimir Putin signaled Moscow’s growing Middle East clout last week on his first visit to Saudi Arabia in over a decade, buoyed by Russian military gains in Syria, strong ties with Riyadh’s regional rivals, and energy cooperation.