Syria has failed to withdraw heavy weapons from cities as required by a United Nations-brokered peace plan, U.N. officials said Tuesday, citing evidence found in satellite images.

[The terms of the ceasefire plan] means withdrawal of all heavy armory [weapons] from population centers and [sending them] back to the barracks. They [Syrian authorities] are claiming that this has happened. Satellite imagery, however, and credible reports show that this has not fully happened, so this is unacceptable, said Ahmad Fawzi, a spokesman for special envoy Kofi Annan in Geneva.

Speaking in Sweden, Annan called on Syria to implement the terms of the ceasefire plan in their entirety.

Fawzi also charged that Syrian soldiers and security forces are harassing and even killing people who were seen talking to U.N. truce observers in the country.

“When they [are there] the guns are silent. We have credible reports that when they leave, the [shelling] start again,” Fawzi said.

There are currently only a handful of U.N. observers in Syria, although there are plans to authorize up to as many as 300.

Syria has a population of about 23 million.

“With 11 or 12 monitors, you can’t be everywhere, and there are many cities that have seen destruction and have seen fighting, and we have to be present,” Fawzi said. “With up to 300, we will be able to monitor more cities than two to three at a time.”

Reuters reported that after one week of relative calm, 20 people were killed by the Syrian military in the flashpoint city of Hama on Monday, just one day after a group of unarmed U.N. monitors visited the area.

Separately, the state-controlled SANA television channel said three people were wounded on Tuesday when a car bomb exploded near the Yelbagha complex in the central district of al-Marjeh in Damascus. Reuters reported the detonation occurred near an Iranian cultural center. Iran is the chief remaining ally of the Syrian regime.

SANA accused armed terrorists” of carrying out the attack.

SANA also reported that a Syrian intelligence officer, Lt. Col. As'aad Ahmad Ismael, and his brother were killed in Jedaydet al-Fadel, near Damascus.

The U.N. estimates that at least 9,000 people have been killed in Syria since the revolt against President Bashar al-Assad erupted last March.