(Reuters) - Syrian forces killed at least five people on Friday when they fired at demonstrations calling for the downfall of President Bashar al-Assad in the capital Damascus and the northern city of Aleppo, activists said.

The shootings occurred despite a three-week-old ceasefire agreement aimed at ending 14 months of bloodshed in Syria, part of a wider peace plan that international mediator Kofi Annan´s spokesman said was still "on track".

The United States said on Thursday that a new international approach may be needed if the plan fails, accusing Assad of making "no effort" to implement it so far.

Annan is due to brief the Security Council next week.

Assad says he is fighting foreign-backed "terrorists" and his international friends, including in Moscow, point out that rebels too have staged attacks in breach of the ceasefire.

Three people were killed when security forces fired on a demonstration in the Kfar Souseh district of Damascus, and a teenager was shot dead in the poor neighborhood of Tadamun, activists said.

Another person was shot dead in Aleppo, a day after security forces and students with knives attacked a university protest against Assad, killing at least four people in Syria´s once relatively peaceful commercial hub and second city.

The British-based Syrian Observatory for Human Rights said three people were killed overnight in Aleppo, and put the death toll in violence across the country over the last 24 hours at more than 20.

Small groups of United Nations observers have deployed across the country to assess compliance by Assad´s forces and their rebel opponents with the April 12 ceasefire.

Their leader, Norwegian Major-General Robert Mood, has said they have had a "calming effect" in their areas of operation, adding that they cannot be expected to solve Syria´s crisis, in which thousands have been killed since March last year.

Hundreds of people, including civilians, security forces and rebels, have been killed since the ceasefire was agreed.

PLAN ON TRACK

Annan´s six-point plan includes a ceasefire, deployment of observers and free access for journalists and humanitarian aid, leading to talks on an political solution.

"I would say that the Annan plan is on track and a crisis that has been going on for over a year is not going to be resolved in a day or a week," Fawzi said.

"There are signs on the ground of movement, albeit it´s slow and small. There are also signs behind the scenes you don´t see because this mediation effort by definition is conducted below the radar," Fawzi said.

Security was tight across the capital Damascus on Friday, with checkpoints and roadblocks throughout the city.

Roads leading to Midan - site of a suicide bombing last Friday which killed at least nine people - were cut off. Seven buses full of security were parked at one entrance to Midan.

Armed men in plain clothes also manned checkpoints while at one roundabout two soldiers sat behind sandbags manning a machine gun.

YouTube footage from Tadamun, which could not be independently verified, showed troops pointing automatic rifles at youths hurling rocks in the poor district on the southern edge of Damascus. Activists said 17-year old protester Mohammad Darwish was killed by army gunfire.

Mood, the chief U.N. observer, said in Homs on Thursday that his mission was steadily growing, with a total of 50 monitors in the country which would be doubled within weeks.

"We have reinforced our permanent teams in Hama and Deraa with an extra two monitors in each city," he said from the al-Safir hotel in Homs, where six monitors are based permanently.

Around 300 monitors will be deployed by the end of May.

Syria´s uprising began in March 2011 with peaceful demonstrations inspired by a wave of Arab revolts against long-ruling autocratic leaders, but it has become increasingly militarized in response to Assad´s violent crackdown.

The U.N. says more than 9,000 people have died in the crackdown, while the Syrian government says it has lost at least 2,600 of its forces to "foreign-backed terrorists".

Despite the turmoil, Syria plans to hold a parliamentary election on Monday under a new constitution which has allowed the creation of new political parities and formally ended decades of monopoly by Assad´s ruling Baath Party.

Authorities say the election is part of a reform process, but the opposition dismisses it as a sham.

(Additional reporting by Khaled Yacoub Oweis in Amman and Stephanie Nebehay in Geneva, writing by Dominic Evans) (© Thomson Reuters 2012. 05/04/12)