Iran said Monday that it had agreed to a deal to deepen military cooperation with Syria, reaffirming its intention to remain in the country despite moves by the U.S. and its allies to contain Tehran’s military reach.

Iran and Syria reached the deal following meetings Sunday between their defense ministers and Syria’s President Bashar al-Assad, according to Iran’s semiofficial Tasnim News Agency. The report offered no further details. Syrian state-linked media said the two sides reaffirmed the need the develop long-term cooperation, but made no mention of a new deal.

Iran has sent thousands of its own and allied forces to Syria to fight alongside Mr. Assad’s troops, and has also supported the Syrian regime with weaponry and oil.

The move to further cement the partnership comes as the Trump administration and Israel amplify calls for Iran to withdraw completely from Syria. Beating back Tehran has been a central goal of U.S. foreign policy under President Trump, who withdrew the U.S. in May from the Iran nuclear deal and has started reimposing sanctions that had been lifted under the Obama-era pact.

Iranian forces have been involved in Syria’s civil war since 2013, coming to the aid of Mr. Assad as he fended off a challenge from rebel groups and Islamic State extremists. Russia intervened to support Mr. Assad in 2015, helping him gain the upper hand in the conflict, which has now stretched more than seven years.

With Mr. Assad tightening his grip, the U.S. and Israel are worried that Iranian influence is now entrenched in Syria. Mr. Trump wants to prevent Iran developing ballistic missiles and expanding a military footprint that could threaten Israel or other U.S. allies in the region.

In a sign of how seriously Israel takes the threat, Israeli missiles struck Iranian targets in Syria in May, its largest assault inside Syria in about seven years. Israeli officials said the strikes destroyed dozens of Iranian military targets and Syrian anti-aircraft installations.

Last week, White House national security adviser John Bolton said Russian President Vladimir Putin believed Russian and Iranian interests in Syria weren’t aligned, “and that [Mr. Putin] would be content to see Iranian forces all sent back to Iran,” recounting a conversation between Mr. Putin and Mr. Trump.

But Mr. Bolton said Mr. Putin indicated he couldn’t evict Iran from Syria by himself, suggesting the Russian president’s leverage over Mr. Assad had limits.

Moscow has previously offered to keep Iran and its allied militias tens of miles from the Israeli border, but its ambassador to Israel has stressed that Russia won’t work to evict Iran from Syria. Iranians play a “very, very important role in our common and joint effort to eliminate terrorists in Syria,” Anatoly Viktorov told Israeli media in July.

Mr. Assad has previously said that Iran’s presence in Syria wasn’t negotiable and wouldn’t be part of a political settlement over the country’s future.

After the Iran-Syria meetings Sunday, Tasnim’s report quoted Iranian defense minister Brig. Gen. Amir Hatami as dismissing “rumors and negative propaganda” about Iran-Syria ties and asserting that they were on a strong footing.

Syria was moving out of a crisis and into a reconstruction phase, he said. Iran’s private sector “enjoys good capabilities and can use them in the reconstruction of Syria,” Gen. Hatami added.

Yet, while Iran has sought economic benefits after investing billions of dollars in the Syrian conflict, it has so far struggled to profit. Its reconstruction efforts are concentrated in companies linked to the elite Revolutionary Guard, and private Iranian companies are reluctant to get involved, partly because of security concerns.

Memorandums of understanding to repair a power grid, run a phone network and partake in extraction from one of Syria’s lucrative phosphate mines have failed to materialize. Iranian companies have purchased some real estate in Syria, but most development so far seems to have taken place in neighborhoods around Shiite shrines where Iran already had a significant presence before the war.

Corrections & Amplifications 
Iranian companies have purchased some real estate in Syria. A previous version of this Aug. 27 article said that Syrian companies had done so.