AMMAN (Reuters) - The Syrian army and allied troops laid siege to the rebel-held enclave in Deraa on Monday and were poised to gain complete control of the city where the uprising against President Bashar Assad’s rule first erupted, insurgents said.

With its advance on rebel-held parts of Deraa city, the government appeared to be piling military pressure on the insurgents even after they agreed to surrender terms last week, in a major victory for Assad.

The army was also consolidating its grip over the border area with Jordan to the east of Deraa city on Monday. Free Syrian Army rebels (FSA), once backed by the West and Jordan, have mostly handed over the area along with their heavy arms to the government since the surrender deal clinched last Friday.

Backed by Russian air power, the Syrian army and its militias scored a strategic victory in a 20-day offensive after they captured Nassib crossing, a vital trade route that insurgents held for three years..

Abu Shaima, a spokesman for the opposition in Deraa, said several thousand people were now encircled after the army pushed into a base west of the city without a fight. “The army and its militias have besieged Deraa completely,” he told Reuters.

State media said the army was fanning out along the border areas with Jordan and tightening the noose on what they term as “terrorists”, but made no mention of the siege of Deraa.

The return of Deraa to Assad’s complete control would deal a big psychological blow to the opposition since the city came to epitomize the early peaceful protests against authoritarian rule in 2011 that spread across Syria. The protests were violently crushed and paved the way for the civil war.

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SURRENDER DEAL

The surrender accord reached on Friday between Russian officers and rebel representatives covers Deraa city along with other towns in the southern province, which borders Jordan.

Before the deal, many towns and villages in Deraa province had been forced to agree to return to state control after a major Russian aerial bombing campaign on urban centers that led to the largest displacement of civilians in the more than seven- year-long conflict.

Opposition sources said the territorial sweep into Deraa province will allow the army for the first time to take over FSA front lines with Islamic State militants, who occupy the Yarmouk Valley pocket southwest of Deraa along both the Israeli and Jordanian borders.

“The Russian military police and the army entered Tafas town and secured a corridor through opposition territory to a front line with Daesh,” a regional intelligence officer said, using the Arab acronym for Islamic State.

As part of the surrender deal, opposition fighters not ready to make peace with the army must first be allowed to evacuate to opposition-held areas in northern Syria before the handover of weapons and the return of state sovereignty.

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“There are fighters who want to go to (opposition-held) Idlib but this was rejected after we were besieged,” said Abu Shaima, referring to a meeting on Sunday in which he said a go-between with the Syrian army had flatly rejected their demands to leave.

The rebels say the deal also does not allow the army to move into their bastions and allows for setting up local forces from ex-rebels under the oversight of Russian military police.

“There is a lot of fear about the unknown fate that awaits us and we do not trust the Russians or (Damascus) regime,” Shaima said, adding that remaining rebels in Deraa city were still holding their front line positions.

Another opposition negotiator said a further round of talks with Russian officers was planned on Monday over the fate of Deraa and security arrangements once it returns to state rule.

“We will work with the Russians on setting up a local force from the inhabitants that will prevent the entry of the army to Deraa with Russian guarantees,” Abu Jihad, a negotiator, said.

The United Nations said on Monday it would immediately start providing humanitarian assistance to thousands of civilian families affected by the fighting in the Deraa, Sweida and Quneitra areas of southern Syria, after the Syrian government asked it to do so.

“The living conditions of the civilians affected by the conflict in the Syrian south are currently dire,” said United Nations humanitarian coordinator in Syria Ali Al Zaatari, and he welcomed the government’s request to mobilize aid.

The offensive in southern Syria has displaced more than 320,000 people, the United Nations said last week.