Secretary of State John Kerry, center, walks with Saudi Foreign Minister Prince Saud al-Faisal as Kerry arrives in Riyadh. Saudi Arabia is the seventh leg of Kerry's first official overseas trip. (Jacquelyn Martin/AP)

Highlighting a divide with the United States over whether to arm the Syrian rebels, Saudi Arabia’s chief diplomat said Monday that there is a moral responsibility to speed an end to the civil war, including by helping Syrians fight the regime’s “vicious killing machine.”

“Saudi Arabia will do everything in its capacity. We do believe that what is happening in Syria is a slaughter — a slaughter of innocents,” Prince Saud al-Faisal said during a visit by Secretary of State John F. Kerry. “We cannot bring ourselves to remain silent. Morally, there is a duty.”

Saudi Arabia is believed to be sending small arms and perhaps other weapons to Syrian rebel fighters. Saud’s brother, former Saudi intelligence chief Prince Turki al-Faisal, called in January for sending heavy arms such as antitank and antiaircraft weapons.

The Obama administration and the European Union have refused to provide weapons, but they agreed last week to begin sending some direct battlefield support to the rebels, who are fighting the government of President Bashar al-Assad.

The United States will provide only food and medicine for now, arguing that weapons could too easily be diverted to Islamist militants working alongside what Kerry on Monday called the “moderate, legitimate opposition.”

Syria is awash in weapons, some supplied to the government by Iran and Russia. Rebels are capturing government stockpiles to augment arms supplied from other countries. Kerry did not directly answer a question about whether weapons supplied by Saudi Arabia, a key counterterrorism partner, are part of the problem.

“There is no guarantee that one weapon or another might not, at some point in time, fall into the wrong hands,” Kerry said at a news conference with the longtime Saudi diplomat. But the Syrian opposition has demonstrated that it can direct support to the right fighters, and the fighters are making good use of what they are already receiving, he said.

Kerry gave no specifics about who is sending what. Syrian Opposition Coalition leader Mouaz al-Khatib, standing alongside Kerry last week, angrily said that outsiders seem more focused on the Islamists than on the true enemy: Assad.

After meeting with Kerry, Saud said, “For its part, the kingdom stressed the importance of enabling the Syrian people to exercise its legitimate right to defend itself against the regime’s vicious killing machine.”

Saud is the world’s longest-serving foreign minister, appointed in 1975. He is known as a shrewd observer of international dealings and is not shy to tweak the U.S. government in public.

Monday’s joint appearance was cordial. Kerry saw Saud for lengthy meetings over two days that were focused on counterterrorism cooperation and the shared view that Iran poses a threat of developing nuclear weapons.

Kerry did not meet with Saudi King Abdullah II. He saw diplomats from several other Persian Gulf nations who were attending a regional conference, and he had a hastily arranged lunch Monday with visiting Palestinian President Mahmoud Abbas.

The Abbas meeting came in lieu of a visit to Israel and the Palestinian territories on this trip, Kerry’s first as secretary. President Obama is visiting Jerusalem and Ramallah later this month, with Kerry in tow, and the White House saw an interim stop by Kerry as potentially diluting the impact of the president’s visit.

Although Kerry said the United States won’t “plunk down a plan” for Middle East peace on that trip, Obama hopes to jump-start talks.

The United States and Saudi Arabia also have a difference of opinion over the value of continued negotiations with Iran over its nuclear program.

Although both Kerry and Saud stressed Monday that they prefer to use diplomacy to resolve the crisis, Saud made a point of saying that Iran’s nuclear program continues “unabated” while talks continue.

Taking a much tougher line than he did last week, immediately after apparently cordial talks in Kazakhstan, Kerry ruled out negotiation with Iran over any subject apart from its disputed nuclear development program.

He said the window of opportunity for a diplomatic resolution of questions over Iran’s nuclear work “cannot, by definition, remain open indefinitely.”

Iran maintains that it is seeking only peaceful nuclear energy. The United States, Israel and many other nations suspect that the energy program could swiftly be converted to make nuclear weapons if Iran chose to do so.

“There is time to resolve this issue, providing the Iranians are prepared to engage seriously” on international proposals to defuse it, Kerry said.

Iranian officials have called last week’s talks with international negotiators, including the United States, productive, and the session ended with an agreement for more talks. There is a modest package of incentives on the table if Iran curbs aspects of its nuclear work that are most apt to be turned to weapons production.