Kyrgyzstan on Friday said it has prevented two major attacks by militants linked to the Islamic State group, one of which would have targeted a Russian airbase in the ex-Soviet Central Asian country.

The security service said its forces on Thursday conducted a raid in the center of the capital Bishkek, killing four suspected "terrorists" in an hour-long shootout.

Security forces in Kyrgyzstan killed four "terrorists" on Thursday during a rare raid in the center of the capital Bishkek, authorities said. 

"As a result of a special operation, four fighters were liquidated after putting up intense armed resistance," the security forces of the central Asian country said in a statement. 

United Nations General Secretary Ban Ki-moon on Saturday warned of a worsening rights situation across Central Asia, as he ended a visit to energy-rich Central Asia in ex-Soviet Turkmenistan.

Ban said he "heard concerns about the deterioration of some aspects of human rights -– a shrinking of democratic space", during his first trip to the region in five years.

Defense lawyers asked a jury Monday to sentence Boston bomber Dzhokhar Tsarnaev to life in America's "super-max" federal prison, portraying a lost teen controlled by a radical older brother.

"As awful as his crime was, life in prison faced with what he has done is a better choice for everyone," said lawyer David Bruck in his opening statement at the penalty phase of Tsarnaev's trial.

Kylychbek Beksariyev lost two friends to a bloody revolution he hoped would transform ex-Soviet Kyrgyzstan, but five years after the ouster of a reviled leader he says his Central Asian homeland remains mired in poverty and corruption.

"Officials still steal," the sports instructor, 27, said. "I work the same job I worked in 2010. The pay increased 20 percent and the price of bread by 30 percent."

Kyrgyz President Almazbek Atambayev assailed Belarus on Thursday for sheltering members of the Central Asian republic's former ruling family, following the reported murder of a prominent gangland figure in Minsk earlier this month.

Atambayev claimed the February 18 discovery of Kyrgyz-born gangster Almanbet Anapiyaev's corpse was "without doubt" the consequence of a "criminal showdown" in the Belarussian capital, allegedly involving the brother Kyrgyzstan's former authoritarian president Kurmanbek Bakiyev.

Up to four thousand people from Muslim former Soviet Central Asian countries are believed to have joined Islamic State jihadists, a report published on Tuesday said.

Often driven by poverty, some "2,000 to 4,000 have in the past three years turned their back on their secular states to seek a radical alternative," the International Crisis Group said in a briefing on the region.

More than 1,000 people in the Muslim Central Asian nation of Kyrgyzstan protested on Tuesday against cartoons of the Prophet Mohammed in the wake of the attack on Charlie Hebdo in France.

People gathered under the slogan "I love the prophet" for the rally in the impoverished ex-Soviet state's capital Bishkek, lawmaker Tursunbai Bakir Uulu told AFP.

One border guard was killed and two wounded in an armed attack close to Kyrgyzstan's volatile border with Central Asian neighbor Tajikistan, official said Saturday.

Authorities in the ex-Soviet state were probing the incident after unknown assailants opened fire late Friday on a border guard base some 25 kilometers from the frontier, Kyrgyzstan's deputy prime minister Abdyrakhman Mamatiliev said.

Accused Boston Marathon bomber Dzhokhar Tsarnaev is due to make his first public appearance in 17 months when he steps into court Thursday for final preparations for his January trial.

Held in Fort Devens prison hospital around 70 kilometers (44 miles) from the federal court in Boston where he will face trial, 21-year-old Tsarnaev, has not been seen in public since he pleaded not guilty in July 2013.