The United States says an ongoing military offensive in northern Syria has created a "humanitarian crisis".
White House officials have called on the Syrian government to end the use of violence against civilians and give aid agencies access to people in need.
More than 4,000 people have fled into Turkey from northern Syria as the army cracks down on months of anti-government protests.
Aid workers have told the BBC the situation at the border is "critical".
"What is absolutely needed is some medical supplies - you have many wounded and you don't have that many hospitals in that remote area of Turkey," said Mans Nyberg of the UN's refugee agency.
"Then of course they will need food, they will need non-food items like blankets, mattresses, more tents."
Clashes across the country after Friday's weekly prayers led to the deaths of at least 32 people.
In a statement, the White House said the "Syrian government's offensive in northern Syria has created a humanitarian crisis".
The US called on the Syrian government to "stop this violence" and to give the International Committee for the Red Cross (ICRC) "immediate, unfettered access" to the northern regions.
"If Syria's leaders fail to provide this access, they will once again be showing contempt for the dignity of the Syrian people," said the statement.
EU foreign policy chief Catherine Ashton also condemned the "brutal force against protesters" while UN Secretary General Ban Ki-moon said he was "deeply concerned and saddened" by the civilian deaths.
Syrians arriving in Turkey have mainly been fleeing the town of Jisr al-Shughour, where 120 security personnel were reported to have been killed last week. Local people say the soldiers were killed after a mutiny but the government has blamed the deaths on "armed gangs".
The government says residents asked for military intervention to restore order, but people fleeing the town have described soldiers firing on residents and destroying homes and crops.
"Tanks came from the south after shelling randomly and sending volleys of machinegun fire all over the town. People are still fleeing from the north," one refugee in Turkey told Reuters.
Protests against President Bashar al-Assad began in March and have spread across the country.
Rights groups say hundreds of people have been killed and there are reports the army has been using aircraft to open fire on people.
Syria has prevented foreign journalists, including those from the BBC, from entering the country, making it difficult to independently verify reports from there.
Turkish officials say at least 4,300 people have crossed the border, but the BBC's Owen Bennett Jones, in Guvecci on the Turkish-Syrian border, says the real number of displaced people is probably much higher, because many arrivals have gone unnoticed by the Turkish army.
Many people are arriving with injuries and some have said they are soldiers who have defected.
One man told the AFP news agency he had deserted after being ordered to shoot unarmed civilians in Homs province.
"When we entered the houses, we opened fire on everyone, the young, the old... Women were raped in front of their husbands and children," he said.
Two camps are already housing refugees and a third is under construction.
Senior Foreign Ministry official Halit Cevik said Turkey would deal with the crisis as best it could but may need outside help if the numbers increase.