BBC News with Iain Purdon

The UN special envoy to Syria, Kofi Annan, has indicated that he's gravely concerned about reports that Syrians who speak to UN observers are being

harassed

or even killed by pro-government forces. Mr Annan's spokesman Ahmed Fawzi commented on reports from activists that a number of Syrians had been executed in the city of Hama after the UN team had visited. Mr Fawzi said the reports

underlined

the need to get more observers into Syria as soon as possible. He also said the UN had evidence that Syrian forces had not withdrawn all their heavy weaponry from urban centres.

"The ceasefire is indeed extremely fragile, and we are calling upon the Syrian government to fully implement its commitments under the six-point plan, and this means withdrawal of all heavy armoury (and) from population centres and back to the

barracks

. And they are claiming that this has happened. Satellite imagery, however, and credible reports show that this has not fully happened."

China has urged Sudan and its newly independent neighbour South Sudan to show

restraint

in the conflict over their disputed border. President Hu Jintao urged both countries to respect each other's sovereignty during a visit by the South Sudanese leader Salva Kiir to Beijing. James Copnall is in South Sudan.

Salva Kiir said his Sudanese neighbours had declared war on his country. His remarks came at the beginning of a state visit to China, one of the few countries with influence on both Juba and Khartoum. On Monday, Sudan's President Omar al-Bashir said there was no possibility of negotiations with the South Sudanese. He said talks would be through the gun. The United Nations has condemned an aerial bombardment of Bentiu in South Sudan which took place on Monday. Sudan denies carrying out the raid.