Jose Luis Rodriguez Zapatero, speaking after a meeting with Silvio Berlusconi, his Italian counterpart, said: "These are very serious allegations. The British presidency (of the EU) has asked for explanations from the American administration and we will listen to these explanations, but we want correct and adequate information."
The leaders told a joint press conference that they had no evidence of clandestine operations in their countries, but Berlusconi said that he shared the position of Jack Straw, the British foreign secretary, who has written to Washington asking for an explanation.
The CIA is reported to have used various European airports for its programme of "extraordinary renditions", under which it has allegedly arrested terrorist suspects and handed them over to third countries with dubious records on human rights and torture.
The agency is also reported to have set up secret jails in some European countries where terrorist suspects are interrogated beyond the reach of US anti-torture legislation.
Berlusconi told a reporter he felt he could safely rule out "that something of the kind could be happening on our territory".
Italy hosts several US military bases, one of which was allegedly used to fly a Muslim preacher to Cairo to be handed over to Egyptian authorities after he was reportedly abducted by the CIA in Milan.
An Italian prosecutor has requested the extradition of 22 alleged CIA operatives to stand trial in Italy, but the justice ministry has not passed on the request to the US authorities.