FACTBOX: Key facts about Basque separatist group ETA

(Reuters) - Armed Basque separatist group ETA said it will end its 15-month-old ceasefire at midnight on Tuesday and warned Spain's government of new attacks.

Following are some facts about ETA:

* WHAT IS ETA?

-- ETA (Euskadi ta Askatasuna, or Basque Country and Freedom) is fighting for an independent Basque state in northern Spain and southwestern France.

* YEARS OF MAIN ACTIVITY:

-- The group has killed more than 800 people since 1968, typically using car bombs or shootings. The number of ETA killings had fallen from 23 in 2000 to three in 2003. A car bomb at Madrid airport on December 30, 2006 which killed two people, was the first time ETA had killed anybody since May 2003.

-- Spain, the United States and the European Union have listed ETA as a terrorist organization. Former Spanish Prime Minister Jose Maria Aznar, who survived an ETA attack while he was opposition leader in 1995, made eliminating the group a priority. His socialist successor, Jose Luis Rodriguez Zapatero, and his government have taken a softer line on ETA and have publicly fostered hopes of a truce.

-- In 2002, the Spanish parliament passed a law which effectively banned the Basque political party Batasuna, which it described as ETA's political wing. The party denies the allegation.

* PRESSURE BUILDS FOR PEACE:

-- ETA has been under pressure in recent years with more than 750 suspected members detained since 2000. A raid in France in 2004 rounded up ETA's "top leader", Mikel Antza.

-- On March 20, 2006, ETA claimed responsibility for a spate of recent attacks, including five bombs planted on northern Spanish motorways earlier that month to coincide with a regional general strike called by Batasuna.

-- Two days later ETA declared a permanent ceasefire. In late June 2006, Zapatero told parliament he would seek to start peace talks with ETA.

* RETURN TO VIOLENCE?

-- A car bomb exploded at Madrid's airport last December killing two and forcing Zapatero to break off the peace process.

-- Last February, Zapatero dismissed an offer from Arnaldo Otegi, leader of the Batasuna party saying that ETA would not demand major concessions from Spain to restart peace talks.

-- In April through an interview, ETA offered to make new commitments to the stagnated peace process if Spain stopped its "attacks" in the Basque region. ETA had blamed the Socialist government and the moderate Basque Nationalist Party for the blockage of the peace process.