The United Nations Refugee agency says up to 700,000 Syrians could flee the conflict-ravaged country by the end of the year.

The UNHCR issued an appeal Thursday for nearly $500 million in aid to help “rapidly increasing” numbers of refugees fleeing to neighboring Jordan, Lebanon, Iraq and Turkey.

It says between 2,000 and 3,000 people are crossing those borders each day, and that the number of registered refugees and those awaiting registration has increased more than 600 percent since March.

The call for aid comes after what a Syrian rights group says was the bloodiest day since the 18-month conflict in Syria began.

The Britain-based Syrian Observatory for Human Rights says more than 300 people — mostly civilians — were killed Wednesday in Syria.

The group reported deaths in areas across the country, including Deir Ezzor, Homs, Hama, and in the capital, Damascus. Its reports from witnesses inside the country cannot be confirmed because of restrictions on foreign media.

Syrian rebels detonated two suicide car bombs at President Bashar al-Assad's heavily guarded army headquarters in Damascus Wednesday, killing four security guards and sparking a gunbattle in which an Iranian journalist also died.

Information Minister Omran Zoubi blamed the attack on terrorists, a term the government uses for rebels opposed to Mr. Assad.