CAIRO - The United Nations said Saturday that it was suspending its observer mission in Syria because of the escalating violence, the most severe blow yet to months of effort by the UN to negotiate a peace plan and prevent Syria’s descent into civil war.

The UN said the monitors would not be withdrawn from Syria, but rather were being locked down in Syria’s most contested cities, unable to conduct patrols.

While the decision to suspend their work was made chiefly to protect the unarmed monitors, the unstated purpose appeared to be to force Russia to intervene and assure that the observers are not the targets of Syrian forces or their sympathizers. Russia has opposed Western intervention and, by some accounts, continues to arm the forces of President Bashar Assad.

For President Obama, the suspension of the observers’ activities - unless it is reversed quickly - could signal the failure of the latest effort by the West to reach a diplomatic solution and ease Assad from power.

But Obama’s choices are no better than they were when the uprising in Syria began nearly a year and a half ago. A bombing campaign like the one conducted last year by NATO in Libya with strong US and Arab League support is not feasible in Syria: The battle is being waged in crowded cities, with little chance to attack the Syrian army without the risk of high civilian casualties.

Obama, NATO nations, and the Arab League have never wanted to send in a ground force, which would probably face heavy casualties in what many fear is emerging as a civil war.

The White House issued a statement Saturday again calling on Syria to uphold commitments it has made in recent months, “including the full implementation of a cease-fire.’’ The statement added, “We are consulting with our international partners regarding next steps toward a Syrian-led political transition’’ called for in two UN Security Council resolutions, and “the sooner this transition takes place, the greater the chance of averting a lengthy and bloody civil war.’’

The Syrian uprising, which began 16 months ago, has become one of the most intractable and deadliest conflicts of the Arab Spring, with reports of at least four massacres in recent weeks, including the accounts of killings of as many as 78 civilians, many of them women and children.

The administration is still resisting calls to arm the disparate rebel groups, for fear that they are not an organized force and that the groups could eventually turn on one another. “The problem is that if we do nothing and Syria explodes, we have a broader conflict in the Middle East,’’ a senior US diplomat said early last week, before the UN announcement, adding, “But our options aren’t any better than they were a year ago.’’

The observers had been the foundation of a six-point peace plan that Kofi Annan, the former UN secretary general and the special envoy to Syria, had sought to hammer out with the consent of Assad and his foreign sponsors, including Russia and Iran.