WASHINGTON - US President Barack Obama on Thursday for the first time called on Syrian President Bashar al-Assad to step down after a crackdown by Syrian forces on demonstrations against the Assad family's 41-year reign.

The Obama administration imposed fresh sanctions on Syria's government, freezing assets in the United States as well as banning petroleum products of Syrian origin.

The latest round of sanctions against Assad and his government prohibit US entities, wherever located, from engaging in any transactions or dealings with Syrian petroleum products.

The moves are intended to ratchet up pressure on Assad, who has used his military and security forces to attack protesters seeking an end to four decades of rule by the Syrian president and his late father, Hafez al-Assad.

As well as the growing Western pressure, Assad also faces criticism from regional Arab states and neighboring Turkey over his escalated military campaign against the uprising since the start of the Muslim holy month of Ramadan on Aug 1.

"Assad is trying to convince Turkey that the attacks have stopped, which could also help appease the US, thinking he could once again stop Washington from calling on him to step down," a Western diplomat in Damascus said.

"But the operations have not even stopped."

Although Syrian authorities have announced the army's withdrawal from Hama and Deir al-Zor, residents say military units are still present in those cities. The army is also still deployed in Homs and the coastal city of Latakia.

Hama residents said Syrian forces raided homes in al-Qusour neighborhood overnight, while hundreds of police and militiamen stormed the Rukn al-Din neighborhood of Damascus. In the southern city of Deraa, where the protests first broke out in March, a resident said tanks and armored vehicles stood at entrances of the city and in main squares around Deraa's the old quarter. Security forces raided homes in the Sabeel district overnight, he said.

In a phone call with Assad on Wednesday, Ban expressed alarm at reports of widespread violations of human rights and excessive use of force by the Syrian security forces against civilians, the UN said in a statement.

"The Secretary-General emphasized that all military operations and mass arrests must cease immediately. President Assad said that the military and police operations had stopped," the statement said.

Referral to ICC

An UN official said last week that nearly 2,000 Syrian civilians had been killed since the protests began five months ago. Syria has expelled most independent media since the unrest began, making it difficult to verify reports from the country.

The UN human rights chief is expected to suggest that the UN Security Council refer Syria's crackdown on protesters to the International Criminal Court, envoys said.

In the besieged port city of Latakia, focus of the latest military campaign, residents said on Wednesday that Syrian forces raided houses in a Sunni district, arresting hundreds of people and taking them to a stadium.

Assad's forces have also attacked al-Raml, a seafront area where a Palestinian refugee camp was built in the 1950s.

Latakia is of particular significance to Assad, from Syria's minority Alawite community. The 45-year-old president, a self-declared champion of the Palestinian cause, comes from a village to the southeast, where his father is buried. The Assad family and their friends control the city's port and finances.

Reuters