BEIRUT—The head of the United Nations mission in Syria said a surge in attacks by both sides in the conflict was impeding the work of its monitors there, as the government´s use of attack helicopters spurred new talk of other options to stop the bloodshed.

With the U.N. mission unraveling and a diplomatic deal appearing unlikely, U.S. and European officials discussed possible next steps with Syrian opposition groups on the sidelines of a two-day conference in Istanbul that ends Saturday, people involved in the talks said.

Opposition leaders said they were pushing for avenues of intervention that would bypass the U.N. Security Council, where Russian and Chinese vetoes would be likely to block a mandate to take action against President Bashar al-Assad´s regime.

Options pursued by the opposition include the establishment of a long- discussed protected zone along Turkey´s border for refugees fleeing violence, according to a diplomat in the region. A so-called safe zone could also serve as a training ground and launchpad for opposition fighters

Discussions of a safe zone have been revived in the wake of mass killings and the first documented sustained use of helicopter gunships by the Syrian government. U.N. monitors observed helicopters firing in the western province of Latakia, and over the towns of al- Rastan and Talbiseh in Homs, said Sausan Ghosheh, spokeswoman for the U.N. Supervision Mission in Syria.

The mission´s head, Norwegian Maj. Robert Mood, said the violence has grown over the past 10 days, "willingly by both the parties, with losses on both sides and significant risks to our observers."

Maj. Mood said that while special U.N.-Arab League envoy Kofi Annan´s six-point peace plan wasn´t being implemented by either side—both of which appeared to instead be "advancing military positions"—there is still no other plan on the table.

The U.N. mission´s Security Council mandate expires on July 21, and must be renewed if the monitors are to stay.

A statement by two senior U.N. officials on Thursday reflected the awareness that the presence of the unarmed monitors was doing little to stem violence. The officials, special advisers to U.N. Secretary- General Ban Ki-moon, called on the international community to consider their "responsibility to protect" in Syria—a principle that allows U.N. member states to step in if a country isn´t shielding its own population from mass crimes. Action doesn´t require a vote by Security Council members.

The advisers, Francis Deng and Edward Luc, called on the international community to take immediate, decisive action using "the full range of tools available under the United Nations Charter," in a statement on the prevention of genocide and the responsibility to protect.

The advisers— cited what they called the Syrian government´s failure to protect its population and a high "risk of further mass atrocity crimes."

The Syrian National Council, the leading opposition body, remains divided on whether international intervention will provoke a bloodier backlash from the government.

But Marah Bukai, a Washington-based member of the council, said she and other dissidents would push the U.S. to act on a responsibility to protect mandate. France has said it would propose a U.N. Security Council vote on a resolution that would authorize an intervention, possibly as part of a military operation, in Syria. Turkish officials have said they are militarily prepared for a safe zone along their border, but need what one regional official called "the ironclad backing of the U.S. and others." in an intervention that would require heavy military protection

"We are entering a new phase, a really bloody phase, and we are going to push for this stronger than ever now," Ms. Bukai said.

Russia on Friday said it had refurbished helicopters for the Syrian military but denied shipping new ones, after Secretary of State Hillary Clinton earlier this week accused Russia of sending Damascus the attack helicopters it was using to bomb restive cities.

The U.N. mission in Syria has come under extreme pressure since the May 25 massacre at the village of Houla. The killings were the start of a pattern of apparently systematic attacks against entire families and even towns, as the Syrian government embarked on a campaign to cleanse opposition strongholds and opposition forces also stepped up attacks.

The work of U.N. monitors since then has also become increasingly dangerous and difficult. On Thursday, a group of about 20 people from the mission were finally able to reach the town of Haffa in Latakia, after more than a week of blocked attempts including an incident on Tuesday in which their convoy came under fire.

In a preliminary report, U.N. observers said they found the town of some 35,000 people all but deserted, with "a strong stench of dead bodies" in the air.

Reports from activist and rights groups this week have also provided new accounts of brutal government tactics in Syria. Human Rights Watch on Friday said it interviewed 10 Syrians, including two women, who described being sexually abused by government forces or witnessing such abuse while in detention.

An Amnesty International report on Wednesday, based on interviews with more than 200 people in northern Syria, documented "the deliberate killings of scores of civilians and the burning and looting of hundreds of houses." —Ellen Knickmeyer in Riyadh contributed to this article. (Copyright © Dow Jones & Company, Inc.) 06/16/12)