By John Davison and Stephen Kalin

BAGHDAD/ERBIL, Iraq (Reuters) - Iraqi forces joined flanks in northern Mosul and drove back Islamic State militants in the southeast on Thursday in a renewed push that has brought them closer to controlling the eastern half of the city.

Forces from the elite Counter-Terrorism Service (CTS) took control of 7th Nissan and Sadeeq districts, linking up with army troops that had pushed through al-Hadba neighbourhood, CTS spokesman Sabah al-Numan told Reuters.

"This is considered contact between the troops of the northern front and CTS. This... will prevent any gap between the axes which the enemy could use," he said by phone. "The enemy is now located only in front of the troops, not at their sides."

Numan said more than 85 percent of eastern Mosul was now under control of pro-government forces, up from nearly 75 percent a week ago.

Brett McGurk, Washington's envoy to the U.S.-led coalition backing the Iraqi offensive with air strikes, training and advice, called the link-up a "milestone" and said in a tweet that Islamic State's defences were weakening.

The campaign to recapture Mosul, Islamic State's last major stronghold in Iraq and the largest urban centre anywhere in the sprawling territory it once controlled, has pushed ahead with renewed vigour since the turn of the year after troops got bogged down inside the city in late November and December.

New tactics, including a night raid, better defences against suicide car bomb attacks and improved coordination between the army and security forces operating on different fronts, have helped forge momentum, U.S and Iraqi officers say.

When it launched the offensive in October, the Iraqi government hoped to have retaken the city by the end of 2016, but Iraqi Prime Minister Haider al-Abadi said in December it could now take another three months to drive the militants out.

2016 DEATH TOLL

Iraq's militarised federal police and rapid response division, an elite Interior Ministry unit, are also battling Islamic State inside Mosul.

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They made gains on Thursday in southeastern districts where advances have been particularly tough.

Rapid response units advanced in the Sumer district, which lies on the eastern bank of the Tigris River, and also in neighbouring Sahiroun, according to a military statement.

Forces have pressed forward much more slowly in that area than troops in the east and northeast which commanders blamed on the militants' hiding among civilians and firing at those who tried to flee.

The ultra-hardline group's loss of Mosul would probably spell the end for the Iraqi side of its self-styled caliphate, which it declared after sweeping through parts of Iraq and Syria in 2014, though militants will likely still be able to wage an insurgency in both countries and plan attacks on the West.

Iraq Body Count (IBC), a group run by academics and peace activists that has been counting violent deaths in the country since 2003, estimated that more than 16,000 civilians were killed in Iraq in 2016, down about 1,000 from the year before.

Around three-quarters of those identified were men, with the rest spilt evenly between women and children, IBC said in a report.

More than two-thirds of the fatalities occurred in the capital province of Baghdad and Nineveh, where Mosul is located, it said.

Reuters could not independently verify the figures.

(Reporting by John Davison in Baghdad and Stephen Kalin in Erbil; Editing by Ralph Boulton)