Pentagon chief Chuck Hagel sought to reassure traditional US allies in a Middle East tour this week but fears over Syria's civil war underscored the upheaval gripping the region.
Hagel's talks on Tuesday in Israel, Jordan and Saudi Arabia were overshadowed by revelations from an Israeli general who alleged Syrian President Bashar al-Assad's regime had used chemical weapons against rebel forces in recent fighting.
"To the best of our professional understanding, the (Assad) regime has made use of deadly chemical weapons against the rebels in a number of incidents in the last few months," Brigadier General Itai Brun, head of the research and analysis division of military intelligence, told a conference in Tel Aviv.
Britain and France also harbour suspicions that deadly chemical agents were unleashed but US officials said they were not convinced and that intelligence agencies were still reviewing the evidence.
The stakes are high as the United States has warned Assad that any use of chemical agents -- or transfer to Lebanon's Hezbollah militants -- would cross a "red line", evoking possible military action.
"It's important that we do whatever we can to monitor, investigate, and verify any credible allegations, given the enormous consequences for the Syrian people," said Hagel's press secretary, George Little.
It was especially crucial to verify the reports "given the President's clear statement that chemical weapons use is unacceptable", Little said in a statement in Riyadh.
US Secretary of State John Kerry said later that Israeli Prime Minister Benjamin Netanyahu had been unable to confirm to him that the Syrian regime had resorted to chemical agents.
The potential fallout from the escalating civil war in Syria -- and Iran's role in the conflict --- has topped the agenda in Hagel's first trip to the region as defence secretary, which began with a three-day visit to Israel.
Hagel came to the Middle East touting an elaborate arms deal with Israel, Saudi Arabia and the United Arab Emirates designed to bolster America's partners as a counterweight to Iran.
The former senator vowed an "ironclad" committment to the Jewish state's security, citing the planned sale of US aircraft and missiles, while trying to play down his past criticisms of Israel.
He also insisted there was no serious rift between American and Israeli leaders over how to prevent Iran from attaining a nuclear weapons capability.
In Jordan on Tuesday, Hagel met Prince Faisal and army chief General Masbal al-Zaben, with the two sides agreeing to "consult closely on a number of issues including chemical weapons and the demands posed by the influx of Syrian refugees fleeing the violence," Hagel's spokesman said.
The Pentagon has reinforced an American military contingent in Jordan, for a total of more than 200 troops, to help prepare for possibly securing chemical weapons sites over the border in Syria.
From Amman, Hagel flew to Saudi Arabia, another old American ally concerned over Arab uprisings and the nuclear project of its regional rival, Iran.
In a dinner meeting with Crown Prince and Defence Minister Prince Salman bin Abdulaziz, the two discussed the preliminary arms accord that will bring advanced missiles to Saudi Arabia's American-made F-15 fighter jets.
The two agreed that granting the Saudis access to the hi-tech weaponry "reflected the close bilateral partnership" between the two countries, the Pentagon said.
The Syrian conflict along with the wider turmoil of the "Arab spring" has generated anxiety in the Gulf monarchies. And the Saudi leadership was stunned when the Americans chose to withdraw support for Egypt's longtime president, Hosni Mubarak, two years ago.
On Wednesday, Hagel heads to Cairo, where US influence has dramatically faded since Mubarak's ouster but America's top brass still retains ties to the country's military.
After talks with his counterpart, General Abdel Fattah al-Sissi, and President Mohamed Morsi, Hagel is scheduled to fly to Abu Dhabi, which has signed up to buy more than two dozen F-16 fighters.