The Syrian army has stopped the Red Cross from entering the Baba Amr region of the Syrian city of Homs to distribute aid.
ICRC President Jakob Kellenberger said the delay was "unacceptable" after a convoy of seven Red Cross vehicles made it to Homs only to be turned back from the region worst affected by almost four weeks of shelling.
Activists have alleged the regime was attempting to remove evidence of executions and other violence in the region before aid agencies were allowed inside.
The UN said it was alarmed at reports of summary executions taking place in Homs and was trying to investigate.
Around 4,000 residents in Baba Amr were left behind after the Free Syrian Army was forced to flee, and now live in fear of a "massacre", said the Syrian National Council.
Earlier 12 people including five children were reported killed in a single mortar attack in the town of Rastan.
Very large protests were also reported across Syria, including in Hama where hundreds gathered to call for the arming of the Free Syrian Army.
Around 9,000 people have been killed in the country since the start of anti-government protests in March 2011, activists claim.
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