BAGHDAD, Iraq (CNN) -

The militant jihadist group Islamic State in Iraq and Syria killed dozens of Syrian troops and workers during an attack on a gas field in central Syria, the Syrian Observatory for Human Rights said.

The Islamic State in Iraq and Syria, or ISIS, killed 270 Syrians, including national guard members, security guards and employees, after storming and seizing the al Shaer gas field in Homs province on Thursday, the group said. The death toll could climb. Another 90 security guards and employees are missing, the anti-goverment activist group added.

Word of the slaughter in Homs came after ISIS violence against Christians and other minority groups in Iraq in recent weeks.

The Islamist militants, now occupying large regions of Iraq and Syria, have issued an ultimatum to the remaining Iraqi Christians in the city of Mosul: accept Islam, pay extra taxes to Islamic Sharia courts, or face "death by the sword."

The letters from ISIS were distributed in recent days to the dwindling number of Christian leaders in Iraq's second largest city.

The message added that ISIS leader Abu Bakr al-Baghdadi has agreed to allow them (Christians who do not agree to convert or pay extra taxes) to leave the northern Iraqi city by noon Saturday (5 a.m. ET). After that, the message said, "the only option is the sword."

But the international organization Human Rights Watch says the extremist group is already "killing, kidnapping and threatening religious and ethnic minorities" in Mosul and other Iraqi cities and towns.

"Being a Turkman, a Shabak, a Yazidi or a Christian in ISIS territory can cost you your livelihood, your liberty, or even your life," Human Rights Watch Middle East Director Sarah Leah Whitson said in a press release issued on Saturday from Duhok in Iraqi Kurdistan.

Human Rights Watch has documented violence against these ethnic and religious minorities, reporting that tens of thousands of families have already fled their communities near Mosul in fear for their lives. Village residents have described horrific attacks by ISIS fighters, who "seize local men and pillage homes and places of worships," Human Rights Watch said.

The militants have summarily executed villagers and Iraqi soldiers alike, slaughtering a group of soldiers in Tikrit last month. Human Rights Watch also said ISIS extremists reportedly killed 40 Shia Turkmen, including children, in four communities in Kirkuk last month.

Militant ISIS jihadists, a Sunni-dominated al Qaeda splinter group, have overrun large parts of Iraq and neighboring war torn Syria over the past months in a violent Islamist insurgency. The militants want to establish an Islamic state, or so-called caliphate, across Sunni areas of both countries.

ISIS already controls hundreds of square miles where state authority has evaporated. It has ignored international borders, establishing a deadly presence from Syria's Mediterranean coast all the way south to Baghdad, making its goal of a caliphate state seemingly within reach.

The magnitude of the crisis is clear from the sharp rise in the death toll over the past two months.

At least 2,400 Iraqis died in violence in June, according to the United Nations Assistance Mission for Iraq. Of those, the United Nations said more than 1,500 were civilians, including 270 civilian police officers, and almost 900 were members of Iraqi security forces.