The Red Cross called on Syria on Friday to allow its aid workers wider access to the civilian population, including people who have been wounded or detained in a military crackdown on popular unrest.
The independent humanitarian agency also appealed to Syrian security forces and protesters to respect human life and treat those arrested in line with international law.
"Despite repeated requests to the Syrian authorities, we have not been granted meaningful access to those in need. We are determined to assist people who are having to cope with the violence," Jakob Kellenberger, president of the International Committee of the Red Cross (ICRC), said in a statement.
"Vital humanitarian assistance must reach vulnerable people without delay," he said.
The Syrian army began a military operation in a restive town near the Turkish border, state television said on Friday, as the country braced for more violent protests against the rule of President Bashar al-Assad.
The top UN human rights official, Navi Pillay, urged Syria on Thursday to halt its "assault on its own people". She said more than 1,100 people may have been killed and up to 10,000 detained since March.
The ICRC and Syrian Red Crescent have carried out short visits to Deraa, Tartous and Homs in the past month, according to the Geneva-based agency. But their limited nature made it difficult to get a full picture of the needs, it said.
"We are reiterating our request for access to all persons detained in order to assess the conditions in which they are being held and the treatment they are receiving, and so that we can confidentially share our findings with the authorities concerned," Kellenberger said.
The former senior Swiss diplomat said he stood ready to go to Damascus to hold talks with the Syrian authorities.