Spain's Interior Minister Alfredo Perez Rubalcaba has said the country cannot trust the truce declared by ETA and it will continue to pursue the armed group's members.
Critics said the ceasefire, which was announced yesterday, is an attempt by the armed Basque group to legitimise its political arm Batasuna ahead of municipal elections in 2011.
However, Mr Rubalcaba said that Batasuna must break with ETA or have them disarm before this can happen.
Mr Rubalcaba said: 'It could well be a step-by-step strategy. (Batasuna) have to get the message: either they break definitively with ETA or they convince ETA to definitively stop its violence.'
ETA has killed over 850 people in its attempt to create an independent state in northern Spain and southwest France.
However, it has recently been weakened by arrests of its members and a rise in support among Basques for a democratic solution to the independence movement.
'The announcement clearly intends to hide their weakness, because if they do not hide their weakness, the calls within their own grassroots support for them to disarm will grow,' the Interior Minister said.
ETA has broken ceasefires several times in the past, most recently in 2006 when a truce was ended by a bomb attack at Madrid's airport, which killed two people.
Past ceasefires have been seen by analysts as attempts by the organisation to regroup with a view to launching further attacks.
Francois Heisbourg of the Paris-based Foundation for Strategic Research said: 'It looks more like a breathing spell of indeterminate duration rather than something indicative of an actual readiness to engage in (a real peace process).
'If the latter were true, underground assurances would have been given and the ceasefire would probably have been configured differently in terms of the symbolism.'
The three masked ETA members who announced the ceasefire in a video did not say whether the truce was permanent or why they had decided to stop the attacks.
A number of Spanish newspapers have criticised the ceasefire as a political ploy ahead of the municipal elections.
Conservative daily paper ABC went with the headline 'ETA offers a farce of a ceasefire to gatecrash the municipal elections'.
El Mundo wrote: 'If the separatist organisation chose this ambiguous and limited formulation at this moment, it is without a doubt because it needs to gain time, both because of the police pressure it faced as well as because of the political price violence entails.'
The newspaper called ETA's declaration a 'tactical pseudo-ceasefire' and it recalled that the outfit took advantage of its two previous ceasefires to 'reorganise and rearm'.
Left-wing daily Publico said: 'It is possible that these steps could lead further. Hopefully.
'But for now ETA is trying to save enough time with a statement that gives the bare minimum to allow Batasuna to make new political movements.'
