REYHANLI, Turkey — Three top U.S. officials toured Turkey's border with Syria on Tuesday, even briefly crossing into Syrian territory, in a concerted effort to underscore one point: The United States is throwing its full support behind its NATO ally in its new fight against the Syrian government and its Russian backers.

As part of the tour, U.S. Ambassador to the United Nations Kelly Knight Craft visited a refugee camp and a border crossing, joined by U.S. Ambassador to Turkey David Satterfield and the U.S. envoy to Syria, James F. Jeffrey. Throughout the trip, all praised their Turkish hosts, lauding what they characterized as Turkey’s hospitality toward Syrian refugees in the country and its seamless coordination of cross-border humanitarian assistance.

The visit came amid rising tensions in Syria’s last rebel bastion, with Turkish-backed rebel groups fighting the Syrian army and its allies’ latest and most serious attempt to retake the northwestern province of Idlib and its surrounding areas. On Thursday, an airstrike killed 36 Turkish soldiers, and Turkey blamed the Syrian government.

The deaths prompted a sharp response from Turkey, which shot down two Syrian warplanes Sunday and inflicted heavy losses on the ground against the Russian-backed troops. Turkey shot down a third jet Tuesday, while Syrian state media reported that it had downed a Turkish drone, the fourth this week.

Turkish President Recep Tayyip Erdogan is set to meet with Russian President Vladimir Putin on Thursday in Moscow to discuss the Idlib escalation. The nonstop airstrikes by Syrian planes on civilian areas have caused the worst humanitarian crisis in the country since the beginning of its nine-year conflict. Idlib, long a haven for people displaced from other parts of Syria, is overflowing with more than 3 million inhabitants, many of whom have been displaced multiple times already.

People in the northwestern corner of Syria, in large part controlled by the Islamist Hayat Tahrir al-Sham group, have nowhere to go: Turkey, struggling to deal with the more than 3 million Syrians living on its territory, has long closed its border to refugees. In 2015, Turkey was the first stop on the migrant route to Europe, but the country closed its frontier with Greece the following year after striking a deal with the European Union.

Following the deaths of its soldiers last week, and in the absence of assistance by NATO, Turkey announced that it is reopening its borders with Greece, leading migrants to flood the border. But the Turkish frontier with Syria remains shut.

At the Syrian side of the Bab al-Hawa crossing, Craft posed in front of a “Welcome to Syria” sign with members of the White Helmets, a civil defense organization that rescues people from rubble after Syrian and Russian airstrikes.

Earlier, at a U.N. site for loading trucks with humanitarian supplies, Craft praised Turkish efforts to deal with refugees and pledged $108 million to U.N. agencies, partly meeting agencies’ request for $500 million in emergency aid to deal with the humanitarian crisis brought on by the renewed Idlib offensive.

“Humanitarian aid is only a response,” she said. “The real answer is an immediate cease-fire, a durable cease-fire.”

Jeffrey and Satterfield also reiterated that the United States stands by its Turkish allies. Jeffrey said that in addition to humanitarian assistance, the United States is willing to provide ammunition to help Ankara in its standoff with Damascus and its “evil” backer Russia.

“We’ve got to ensure they have what they need” in Syria, Jeffrey said. “This is a NATO ally. And the Turks laid out a number of requests of NATO. We’re pressing NATO to be responsive.”

Satterfield said the aim of the offensive by Russia and Syria is clear: to drive refugees toward Turkey, “knowing that this is for Turkey a self-identified existential challenge.”

The offensive, Jeffrey said, is bringing about “a degree of human misery we haven’t seen anywhere. This is already the biggest humanitarian crisis, and it’s being deliberately weaponized against Turkey, to push Turkey out of the war and ensure a military victory for” Syrian President Bashar al-Assad.

The U.S. official disparaged what he described as Russia’s “phony” cease-fires — the Russian and Turkish leaders have repeatedly struck cessation-of-hostility deals for the Idlib area, only for them to be broken by Syrian ground or aerial attacks.

But the United States has yet to heed Turkey’s call for Patriot missiles: The two countries faced off last year after Turkey’s decision to purchase S-400 air defense systems from Russia, which led the United States to remove Turkey from the F-35 program.

Asked whether the United States would respond to Turkey’s request, Satterfield said: “We understand the Turkish request for air defense. We are examining that request. And I have nothing further on that.”