The international aid group Doctors Without Borders, also known as Medecins Sans Frontieres, called Tuesday for international donors to support humanitarian operations in opposition-held areas in Syria.

The group said in a statement issued ahead of a donors conference in Kuwait City that areas under government control receive nearly all international aid.

"The current aid system is unable to address the worsening living conditions facing people inside Syria," said Dr. Marie-Pierre Allie, president of MSF. "The participants in the Kuwait City conference must acknowledge the legitimacy of cross-border humanitarian operations intended for Syria and grant them the financial, administrative and logistical support they require."

The number of refugees registered with the United Nations jumped by 110,000 in January, bringing the total to more than half a million.

If the refugees awaiting registration are included, the number exceeds 700,000, the United Nations High Commissioner for Refugees said.

"It's an unrelenting flow," UNHCR spokeswoman Sybella Wilkes said in Geneva.

When Syrians flee to neighboring countries, they arrive in need, Wilkes said.

"The last thing that Syrians wanted to do was leave their country," she said, adding that it's not unusual for families to be displaced internally six times before finally deciding to leave their home country.

They often require with health conditions that have gone untreated and require urgent attention, Wilkes said.

Recently, one woman lost a child at a U.N. station shortly after delivery because she had not received basic prenatal care, Wilkes said.

The exodus of refugees accelerated in the second half of 2012, Wilkes said. A year ago, the United Nations had registered fewer than 20,000 refugees from Syria, which has a population of more than 22 million. Six months ago, the number rose to 120,000. By December, the number had climbed to 471,000. And now, nearly 585,000.

Jordan, Turkey and Lebanon are sheltering the most Syrians, with more than 150,000 each.

"We have in Jordan day shifts and night shifts" to register new refugees, Wilkes said. "In Lebanon, we have double shifts."

U.S. humanitarian aid

U.S. President Barack Obama has approved $155 million in humanitarian aid for people in Syria and refugees fleeing the violence.

That brings the aid total to $365 million, making the United States the largest single donor of humanitarian assistance to Syria.

The U.S. government has imposed sanctions against Syria, worked to isolate the regime, and backed the opposition but has refrained from intervening militarily.