The regime is trying to wrest territory away from rebels who have been able to establish growing enclaves in northern Syria and control much of the main western highway from Aleppo to the Turkish border.

An estimated 200,000 people in and around Aleppo fled their homes over the weekend, the U.N. refugee agency said Tuesday, citing the International Committee of the Red Cross and the Syrian Arab Red Crescent.

"With armed violence raging in Syria's most populous city, thousands of frightened residents are seeking shelter in schools, mosques and public buildings," the agency said in a briefing.

People calling the U.N. High Commissioner for Refugees office in Damascus report "a lack of safety," "fear of ongoing shelling" and a "lack of access to food, water and sanitation, the agency said.

The United Nations says it has registered more than 129,000 Syrian refugees in Turkey, Jordan, Lebanon, Iraq, Algeria, Morocco and Egypt. The refugees include "growing numbers" from Aleppo who "are fleeing across the Hatay border" into Turkey.

The U.N. refugee agency says thousands of people that it hasn't registered have entered those nations.

For example, the Jordanian government said that about 150,000 refugees have entered Jordan since March of last year, when the conflict began. But the United Nations said more than 38,000 are getting protection and assistance, and the rest aren't registered.

Only 70 Syrians have approached the U.N. refugee office for help in Algeria, but there are reports of 10,000 to 25,000 Syrians in Algeria, the refugee agency said Tuesday.

Many Iraqi refugees who took refuge in Syria because of violence in their homeland are returning. More than 20,000 people have returned home in the past 10 days.

The unrest in Syria started when al-Assad's security forces launched a violent crackdown on peaceful protests in March 2011. That clampdown spurred a nationwide uprising and led to the appearance of armed rebels, such as groups of military defectors and other fighters battling under the rubric of the Free Syrian Army.

The conflict has claimed almost 17,000 lives, U.N. Secretary-General Ban Ki-moon said last week. Opposition activists put the toll at more than 20,000.

The violence has been decried across the globe, and it is taking its toll in the inner circles of government. A recent bombing at a government building in Damascus killed four top Syrian officials, and several diplomats have abandoned the government.

The top diplomat at the Syrian Embassy in London resigned his post, the British Foreign and Commonwealth Office said Monday. Charge d'Affaires Khaled al-Ayoubi told British officials that he was "no longer willing to represent a regime that has committed such violent and oppressive acts against its own people," the office said in a statement.