By Sugam Pokharel, Manveena Suri and James Griffiths
Indian paramilitary soldiers patrol a Maoist stronghold. Left-wing insurgent groups are active in at least 13 states across India.
India's decades-long Maoist insurgency has claimed more lives in what Prime Minister Narendra Modi denounced as a "cowardly attack."
At least 25 police officers were killed and six others injured Monday when hundreds of suspected Maoist rebels attacked a convoy in central India, officials told CNN.
Since 2010, Maoist rebels have killed about 2,100 civilians and 800 security force personnel as part of their ongoing conflict with the Indian government, according to official figures.
Indian Maoist -- or Naxalite -- groups have been active in the country since the 1960s, but the modern insurgency did not begin until the early 2000s with the emergence of the Communist Party of India (Maoist) and its armed wing, the People's Liberation Guerrilla Army.
According to police superintendent Jitendra Shukla, around 70 members of India's Central Reserve Police Force (CRPF) were patrolling a road construction project in Chhattisgarh state's Sukma district when they were ambushed.
The security forces were hit by heavy gunfire from about 500 Maoist rebels hidden in the dense forests they were passing through, CRPF spokesman Siroj Kujur told CNN.
Kujur said 23 officers died at the scene, and two died later in the hospital. Six officers were also injured in the attack.
Prime Minister Modi said the attack was "cowardly and deplorable."
"We are monitoring the situation closely," he said on Twitter. "The sacrifice of the martyrs will not go in vain. Condolences to their families."
This is the second attack on CRPF troops in Chhattisgarh in less than two months. In March, 12 officers died in a suspected Maoist attack on another convoy in the same district.
The death toll in Monday's incident compares to a 2013 attack on a government motorcade that killed 24 and injured more than 30.
