Islamist militants have seized a gas field in Syria, killing more than 250 people, according to a Syrian human rights group.

The Syrian Observatory for Human Rights said the dead included staff and security guards at the al-Shaer gas field in central Syria.

It blamed the attack on the militant jihadist organization Islamic State in Iraq and Syria (ISIS).

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The human rights watchdog, which is critical of the Syrian government, put the death toll in the gas field attack at 270, but added the number could rise as another 90 employees and security guards were missing.

The news followed reports of ISIS violence against Christians and other ethnic minorities in recent weeks.

ISIS, an al Qaeda faction, dominated by Sunnis, have seized control of large parts of Iraq and Syria in a violent campaign to set up an Islamic state in the predominantly Sunni areas of both countries.

In the Iraqi city of Mosul, ISIS has threatened to expel or execute Christians unless they convert to Islam or pay additional taxes to Sharia courts.

In an ultimatum that has been circulating in recent days in Mosul, militants gave Iraqi Christians who refuse to accept Islam or pay taxes a Saturday noon (5 a.m. ET) deadline to leave the city or face punishment by death.

According to Human Rights Watch, ISIS fighters have forced thousands of families out of their homes over the past months.

It added that in an attack last month in Kirkuk, 40 Shia Turkmen, including children, died.

Sarah Leah Whitson, the director of Human Rights Watch for the Middle East, highlighted the dangers that ethnic minorities faced in Iraq and Syria, in a statement issued to the media on Saturday. She said that "being a Turkman, a Shabak, a Yazidi or a Christian in ISIS territory can cost you your livelihood, your liberty, or even your life."

TagsEthnic minorities in Iraq, Iraqi Christians