BEIRUT (Reuters) - The Syrian army intensified its onslaught in eastern Ghouta on Saturday with advances that a war monitor and state media said had splintered the enclave, though a rebel official denied this.

Syrian state television broadcast from inside Mesraba, a town lying along the road connecting the northern and southern halves of the rebel-held stronghold.

The capture of Mesraba and advances into nearby farmland brought important roads directly under fire by the army, the Syrian Observatory for Human Rights said.

That has in effect cut the large towns of Harasta and Douma off from each other and the rest of the enclave, it added.

However, Hamza Birqadar, a spokesman for Jaish al-Islam, one of the two main insurgent groups in eastern Ghouta, denied that either Harasta or Douma had been cut off.

The relentless three-week assault on the last major rebel stronghold near Damascus has captured about half its area and killed 976 people, according to the Britain-based Observatory.

State television showed a huge plume of dark smoke rising behind houses and trees in eastern Ghouta, with the sound of blasts in the background.

Syrian President Bashar al-Assad and Russia, his main ally, say the campaign is needed to end rebel shelling of Damascus and end the rule of Islamist insurgents over the area's civilians.

The offensive follows the pattern of previous assaults on rebel strongholds, deploying air power and tight sieges to force insurgents to accept "evacuation" deals.

These involve rebels surrendering territory in exchange for safe passage to opposition areas in northwest Syria, along with their families and other civilians who do not want to come back under Assad's rule.

Late on Friday, a small number of fighters and their families from the former al Qaeda affiliate previously known as the Nusra Front left eastern Ghouta under such a deal.

But the group represents only a small portion of the insurgent presence in the enclave. Jaish al-Islam and the other main eastern Ghouta rebel group Failaq al-Rahman have said they are not negotiating a similar deal for themselves.

Capturing the enclave would represent Assad's biggest blow against the rebels since they were driven from Aleppo in December 2016.

It would seal a string of military victories for the Syrian leader since the entry of Russian jets into the war on his side in 2015 turned the course of the conflict against the insurgents.

SHORTAGES

The intensity of the government's attack on an enclave that has been besieged since 2013 and suffers acute shortages of food and medical supplies has drawn Western condemnation and demands by U.N. aid agencies for a humanitarian halt in fighting.

State television broadcast from Mesraba showed a large group of civilians hiding in a house. People across eastern Ghouta have sheltered in basements from the ceaseless bombardment in recent weeks.

A middle-aged man interviewed by the channel shouted slogans in support of Assad and against the rebels while women and small children stood behind him as men in army uniforms looked on.

State media said the 60 civilians found in Mesraba by the army had been held by insurgents as human shields, something the rebel groups deny.

Footage taken on the streets of Mesraba showed tanks near half-collapsed buildings and walls pocked with bullet holes.

The United Nations estimates that 400,000 people are trapped in eastern Ghouta.

"Living conditions are harsh ... Shop owners and traders are sending their workers to the shelters to sell food for three times the price before the offensive," said a man in the eastern Ghouta town of Saqba who identified himself as Abu Abdo in a voice message.

Aid agencies have tried to deliver food and medicine into eastern Ghouta, but have been able only to bring in a portion of the amount they wanted.

A convoy was unable to finish unloading on Monday because of continued fighting, bringing in the remaining undelivered food parcels on Friday despite bombardment nearby.

However, U.N. agencies said most medical supplies had been stripped from the convoy by Syrian government officials and that the food supplies brought in were insufficient.

Medical charities operating in eastern Ghouta have reported several incidents in recent weeks of what they say was chlorine gas use in government bombardments, causing choking symptoms.

On Saturday, Syria's deputy foreign minister, Faisal Mekdad, rejected those charges and accused the United States of orchestrating the accusations in order to support militant groups. "We did not deny a single request for investigation," Mekdad told a news conference.

The government has opened what it says are several safe routes out of eastern Ghouta for civilians, but none are known to have left so far. Damascus and Moscow accuse the rebels of preventing people from fleeing the fighting.

Insurgent groups in eastern Ghouta deny this, saying people have not left for fear of government persecution, but a Reuters witness on Friday saw gunfire and mortar fire from inside rebel territory near one of the crossing points.

Sajjad Malik, the U.N. High Commissioner for Refugees representative in Syria, who went in with the convoy on Monday, said in comments published on the agency's website that people did not feel safe to leave.

People in Douma told him they feared crossing rebel checkpoints and were uncertain whether they would be safe when they reached government-held areas, Malik said.

(Reporting by Angus McDowall; Additional reporting by Kinda Makieh in Damascus; Editing by Dale Hudson and Gareth Jones)