The Islamic State (ISIS) has warned that it will escalate terrorist attacks against Christians in the coming weeks, reports said.

It appeared to have carried out the threat on Monday night when a lorry ploughed through a Christmas market in Berlin, Germany, killing at least 12 people and injuring nearly 50 other shoppers, reports said.

Terror gripped shoppers as a huge vehicle smashed into booths outside the Kaiser Wilhelm Memorial Church in the centre of the city. Just an hour earlier, the Russian ambassador was shot dead in Ankara.

ISIS issued its latest terror warning on Dec. 13 after claiming a deadly terrorist attack on Coptic Christians in Cairo, Egypt, which left 25 dead, the Daily Caller reported.

"Let all the disbelievers and apostates in Egypt and everywhere know that our war on polytheism is ongoing, and that the State of the Caliphate – with permission from Allah the Almighty – will continue to spill their blood and grill their bodies, so that there is no sedition and the religion is all for Allah," said ISIS in a statement, according to SITE Intelligence Group. Experts said ISIS regards Christianity—with its belief in the Holy Trinity—as a polytheist religion, or a faith with more than one deity.

ISIS specifically targeted Christian communities in Iraq and Syria in 2014 when it started its rise to power. Its militants have since then been killing Christians to this day. Many of Iraq's ancient Christian communities, particularly in the Nineveh Plains region, are now destroyed, the Daily Caller said. Some of them, like the Assyrian and Chaldean sects, date back to the earliest days of Christianity.

As early as late November, the U.S. Department of State and the British government warned that there was an increased risk of terrorist attacks in Europe during the Christmas season. European holiday markets draw large numbers of people, making them a tempting target for ISIS and other terrorist groups.

Britain's domestic intelligence agency MI5 has warned that a terror attack on British streets is highly likely for the past two years.

A security source told the Daily Mirror: "Britain is very much in the sights of the jihadists but thankfully plots and plotters have been disrupted and stopped

"Whilst U.K. agencies have been extremely good at preventative intelligence operations against jihadist groups, other European equivalents have been less successful, sadly."