The outgoing UN envoy to Iraq has warned the Security Council that Syria's civil war has spilled over into Iraq, saying that "the battlefields are merging" into one conflict, which could destabilize the broader Middle East.

"These countries are interrelated," UN Iraq envoy Martin Kobler said. "Iraq is the fault line between the Shia and the Sunni world and everything which happens in Syria, of course, has repercussions on the political landscape in Iraq."

More than 160,000 refugees have fled the civil war in Syria for Iraq

According to UN figures, nearly 3,000 Iraqis have died in sectarian bloodshed in the past four months, the highest death toll since 2008. Another 7,000 have been injured. And increasingly, Iraqi jihadists and weapons are moving across the border to fight against the regime of President Bashar al-Assad in Syria, UN envoy Kobler said.

"You have the Islamic State of Iraq, that's launching most of the attacks, now operating on both sides of the border and getting stronger and stronger in Syria," Patrick Cockburn, a veteran Iraq reporter for Britain's The Independent, told DW.

"It has bases in eastern Syria right over to the Mediterranean, so that has made the organization much stronger - given it strength and depth," Cockburn said. "It has access to arms depots that it's captured in Syria."

Iraqi Sunnis 'snubbed'

Although the grinding civil war in Syria has heightened tensions in Iraq, Erin Evers with Human Rights Watch said that the government in Baghdad also bears responsibility for the escalating violence in Iraq.

Many Sunnis believe the Iraqi vice president was prosecuted because he was Sunni

"There's been an overstatement of the extent to which what is happening in Syria has affected the increase in violence in Iraq, to the detriment of placing the responsibility with the Iraqi authorities themselves, for failing to meet those demands of Sunni protests that are legitimate," Evers told DW.

Wayne White, a former senior US State Department Iraq analyst, attributed the uptick in violence to Shiite Prime Minister Nuri al-Maliki's failure to include Sunnis in the political process since the US troop withdrawal.

"There was tremendous interest in the Sunni Arab community in essentially ridding themselves of al Qaeda in Iraq and being included and accepting even a secondary role in Iraqi politics," White told DW. "And Maliki essentially snubbed them - aggressively snubbed them - even pursued certain leaders and cadres and killed them and arrested them."

Opposition to Shiite power

White pointed to the Maliki government's prosecution of Iraq's Sunni vice president, Tariq al-Hashemi. In September 2012, al-Hashemi was sentenced to death on terrorism charges, forcing him to flee to Iraq's semi-autonomous Kurdistan region.

Both Sunni protesters and Iraqi soldiers died in the Hawija clashes

The following December, Iraqi Sunnis began protesting against alleged marginalization and persecution at the hands of the Shiite-dominated central government, since the overthrow of Sunni dictator Saddam Hussein by the US in 2003. In the town of Hawija, government troops opened fire on Sunni protesters last April, killing more than 50 people, according to the International Crisis Group.

"[There] is a broad sense amongst many Arabs - not just Iraqi Sunnis, but Sunnis in Syria and the Gulf and elsewhere - that American imperialism came in and ruined their just hold on power, and they're trying to get it back," Joshua Landis, director of the Center of Middle Eastern Studies at the University of Oklahoma, told DW.

Syria as regional spring board

According to Landis, Iraqi Sunnis got a "second wind" when their Syrian co-religionists rose up against the Assad regime. In April 2013, al Qaeda's branch in Iraq announced that it had joined forces with Syrian Islamists to become the Islamic State of Iraq and the Levant.

"[Iraqi Sunnis] began coming to Syria to join into the jihad, in the hopes that if they could win in Syria, they could then double back with Syrian troops at their heels and take the fight back to these Iraqi Shiites and defeat them," Landis said.

Former State Department analyst White said that Sunnis in Iraq feel bound to Syrian Sunnis. The border between the two countries was arbitrarily drawn during the colonial period, and Iraqi and Syrian Sunnis maintain close tribal and clan ties.

"In this case, there are such close ties between the Sunni Arab community in northwestern Iraq and the Sunni Arab community in eastern Syria," White said. "That there is a very natural geographic avenue for this to become regional in the sense of just Syria and Iraq - that border is very arbitrary."