Christian leaders in Syria said they have lost contact with the Christian community in the southeastern Syrian province of Homs where Islamic State militants are believed to be holding dozens of Christians and other residents following the fall of Qaryatain, a town just outside the provincial capital city of Homs, to ISIS forces.

"We have no communication with them [Christian residents], as land lines and mobile lines are being cut off," said Bishop Philip Barakat, an Episcopal vicar in Homs city.

ISIS militants seized the Christian hostages after they took over Qaryatain on Wednesday in their effort to establish a stronghold outside the city of Homs, their next target.

The Syrian Observatory for Human Rights, a UK-based monitoring group, said more than 230 people were seized, among them at least 60 Christians, although this number can increase by up to 500, according to the human rights group's executive director Rami Abdurrahman.

But residents of nearby towns said the numbers of those allegedly kidnapped could not be verified, adding that some residents may have locked themselves in their homes under orders from ISIS, Fox News said.

Abdurrahman said some of those kidnapped by the ISIS militants were Christians who were taken from the town's Dar Alyan monastery.

Other hostages included those suspected to be loyal to the leadership of Syrian President Bashar al-Assad, whose military forces are currently fighting the jihadists.

The whereabouts of the kidnap victims, including scores of women and children, have yet to be determined, according to the human rights group.

Abdurrahman expressed fears that the next target of the ISIS may be other Christian population centres in Syria, particularly Hawwarin and Sadad.

ISIS forces managed to seize Qaryatain after they attacked Syrian-regime checkpoints by detonating three suicide bombs, sources said.

Qaryatain is considered as a strategic launching point for future ISIS attacks against the Syrian regime. The town's population is made up mostly of Sunni Muslims, with a small Christian minority. It is situated just 25 miles from the International Highway, a road that links Syria's capital Damascus with regime-controlled areas in Homs and the western coast.

If ISIS takes control of the highway, it can potentially cut off Damascus from other areas controlled by the Assad regime, further weakening Assad's power.

In a related development, United States Department of Defense spokesman Capt. Jeff Davis said most Syrian rebels trained by the US to fight the ISIS are currently unaccounted for in northern Syria.

Some are even suspected to have reached as far as Turkey.

Davis said the Pentagon is currently tracking the missing Syrian rebels.

These American-trained rebels, members of the New Syrian Forces, are required to report their movements to the US. They have been provided means to communicate with the US military.