Asked which terror group might be responsible for the blasts, Dr Singh, speaking to reporters on a flight to Frankfurt, said, "We have so far not gotten to the bottom of it."
No group so far has claimed the responsibility for the blasts.
The prime minister, who is on an eight-day visit to Brazil [ Images ] and Cuba to attend the Nonaligned Movement Summit, will also visit Brazil at the invitation of President Luiz Inacio Lula da Silva to discuss bilateral issues.
This will be followed by the India-Brazil-South Africa Summit, with President Mbeki joining the two leaders.
'The IBSA has already emerged as a shining example of South-South cooperation. The first IBSA summit will provide a useful opportunity to review the progress that has been made over the last three years since IBSA was first established,' the prime minister said in a written statement.
He further said, 'Our discussions are expected to focus on issues of global concerns including energy security, international terrorism and the challenges of sustainable development. An IBSA business summit will add a new dimension to our interactions.'
From Brazil, the prime minister will go to Havana to participate in the 14th NAM summit.
Dr Singh said, "As a founding member of this great movement, India [ Images ] will play its part in helping NAM to revitalise itself so as to pursue the shared interests of its member states in a transformed world.
"Mutual support and solidarity among NAM members is of prime importance as we confront several common challenges such as making globalisation more inclusive, the scourge of terrorism and addressing widespread hunger, poverty and diseases."
He further stated, "We will rededicate the NAM to the universal causes of peace, disarmament and the progress and prosperity of all human kind."
He is likely to meet Pakistan President Pervez Musharraf [ Images ] on the sidelines of the summit.
This would be the first meeting between the two heads of state after the 7/11 Mumbai [ Images ] serial blasts.
Dr Singh however refused to comment on Bharatiya Janata Party [ Images ] president Rajnath Singh's demand that he should not meet Musharraf.
When asked what he point of meeting Musharraf when terror groups and camps were still operation against India from Pakistan, Dr Singh said, "We will cross the bridge when we come to it."