WASHINGTON — As US President Donald Trump and Russian President Vladimir Putin tentatively prepare to meet next month, one of the few tangible successes of US-Russian cooperation since Trump came into office is on the verge of collapse. Syrian army forces, backed by Russian airstrikes, have been advancing against rebels in the southern Syrian city of Daraa, in violation of a southwest Syria de-escalation zone agreement that was negotiated by the United States, Russia and Jordan last year, and heralded until recently by the Trump administration as a major success.

While the State Department has repeatedly expressed deep concern over the escalation and called on Russia and the Syrian regime to honor the cease-fire, the Syrian military operation has advanced over the last week, causing some 45,000 people to flee, according to UN estimates.

On June 19, the US Embassy in Amman, Jordan, sent a stark message to rebels in southern Syria, telling them essentially that the United States would not use military force to protect them from the Russian-backed Syrian regime advance.

“We in the US government understand the difficult conditions you are now facing, and we are still advising the Russians and the Syrian regime not to take any military action that violates the de-escalation zone in Syria’s southwest,” the June 19 message from the US Embassy in Amman to the southern rebels said. (A State Department official confirmed to Al-Monitor that this was an authentic message. A Syrian opposition source said the message was posted by the US Embassy in Amman to recipients on its WhatsApp group.)