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The people of war-torn Sudan learned long ago to take cover when planes roared overhead, but the latest tactic being used on them -- parachute bombs -- is raining silent death down on innocent villagers, say alarmed activists.

The country’s extremist Islamic regime in Khartoum has stepped up the practice in the Nuba Mountains, dropping deadly bombs by parachute from high altitudes as president and accused international war criminal Omar al-Bashir seeks to rout rebel forces opposed to his brand of radical Islam.

In recent years, the Nuba Mountains, where Christians and Muslims live side by side, have become a battleground for the forces of al-Bashir's forces and the Sudanese People Liberation Army.

Caught in the crossfire are innocent civilians, especially children, who live in the mountainous region just north of the border of Sudan and South Sudan, the nation carved out of Sudan in 2011.

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"Children living in the Nuba Mountains grew up amid almost daily aerial bombardment,” Akshaya Kumar, a Sudan and South Sudan policy analyst with the Center for American Progress, told FoxNews.com. “They have learned how to quickly duck into makeshift bomb shelters when they hear a bomb dropping.

"Now, in a brutal shift in tactics, the Sudanese government has refined its assault," she continued. "With parachute bombs, the bombs drop silently and then only explode after a delay, when those sheltering emerge from safety."

On Monday, local aid workers told FoxNews.com two Sudanese Air Force jets dropped 13 parachute bombs on the villages of Tamadirgo and Dar, in the South Kordofan States of Sudan. The bombs killed at least three people, including a 13-year-old boy, sources said.

Al-Bashir, the 70-year-old dictator and former Army general, seized power in a 1989 military coup. As he moved the nation toward Islamic rule in the late 1990s and early part of this century, rebels fought back against the marginalization of Christians. Al-Bashir responded with a campaign of ethnic cleansing in the western region of Darfur, with an estimated 500,000 people killed and more than 2 million displaced. In 2008, al-Bashir was indicted by the International Criminal Court for genocide and crimes against humanity.

Al-Bashir's embrace of parachute bombing is the latest demonstration of his willingness to kill his own people, according to aid workers. Graphic footage provided to FoxNews.com of a Dec. 10 drop shows mountain villagers cowering as the parachutes flutter downward, then screaming in anguish as they tend to their dead.

“Some people ran away as soon as they saw the plane, while others stayed out of curiosity; they thought that they were parachuters landing,” Ahmed Khatir, who is from the region and works as a reporter for independent news service NubaReports.org, told FoxNews.com. “When the bomb got closer [to the ground], they realized it was a bomb and it was too late for some people."

Since April 2012, 1,371 bombs have been dropped on civilian targets in Nuba, according to NubaReports.org. But the parachute tactic only began in November, dropping bombs that weigh up to 820 pounds. With a delayed detonation and quiet drop, the parachute bombs have proven destructive and deadly.

The Sudan Consortium has recently reported that human rights monitors found that 22 civilians in South Kordofan were killed and 41 seriously injured in a four-week period in December and January. Monitors also documented 56 bombing attacks -- a number that tripled from the previous month. The recent numbers are the highest ever recorded by the consortium.

One apparent reason for the escalation of violence in the Nuba Mountains is new political developments in Darfur, South Kordofan, and Blue Nile going largely unnoticed as the international community has focused their attention on violence in the South Sudan.

"The difference in Sudan is the aerial terror sown by the Sudanese Air Forces," Enough Project co-founder John Prendergast told FoxNews.com