By Khaled Yacoub Oweis

AMMAN (Reuters) - The Syrian army has stepped up an offensive on opposition Sunni Muslim strongholds in the central city of Homs, bringing in ground forces and loyalist militia to try to secure a major road junction, opposition sources said on Friday.

Around 15,000 Sunni civilians are trapped on the southern and western edge of the city near the intersection of Syria's main north-south and east-west arteries, crucial to let the army travel between Damascus and the Mediterranean coast, opposition campaigners in Homs said.

Rebels said they had moved into new areas of Homs this month to grab more territory, which could explain the offensive. Activists said that rebels had asked them not to report on the advances because it could provoke retaliatory strikes.

But activists in Homs said a barrage of army rocket, artillery and aerial bombardment had killed at least 120 civilians and 30 opposition fighters since Sunday.

In the south, eight members of Syria's military intelligence were killed by an Islamist militant car bomb on Thursday night near the frontier with the Israeli-occupied Golan Heights, opposition activists and a violence monitoring group said on Friday.

The Syrian Observatory for Human Rights said the bomb was planted by Al-Nusra Front, a rebel unit fighting to oust Assad that the United States has labeled a terrorist group.

"We think the blast might have killed a colonel who has been leading the fight against rebels in the area," Rami Abdelrahman, head of the Britain-based Observatory said. The building targeted is in the town of Saasa, 14 miles (23km) from the frontier with the Golan Heights, he said.

Syrian authorities have banned most independent media, making it difficult to verify such reports on the ground.

The nearly two-year-old conflict has now killed an estimated 60,000 people and a military stalemate has formed while hundreds of thousands of refugees flood into Syria's neighbors.

The Syrian Interior Ministry called on Thursday for Syrian refugees to come home and said they would be guaranteed safety.

A statement on the state news agency SANA said the government was "offering guarantees to all political opposition sides to enter the country ... (and) ... take part in the national dialogue without any query."

Few who left have returned, especially opposition supporters, and Assad said in a speech this month that he would not talk with opposition members he said had betrayed Syria or "gangs recruited abroad that follow the orders of foreigners".

The war has reached every province in the country and fighting has encroached on the heart of the capital Damascus, with residents reporting the daily thud of artillery being fired on rebel-held districts in the outskirts.

U.S. ambassador to Syria Robert Ford told CNN on Thursday that Assad's mother Anisa Makhlouf and his sister Bushra had both moved to the United Arab Emirates. It is not clear why they left. Continued...