"This attack is yet another serious blow to the ceasefire agreement and the peace process," the Scandinavian-run Sri Lanka Monitoring Mission said in a statement.
It said Tuesday's bombing in Colombo's army headquarters was likely to have "very negative effects" on the relationship between Colombo and the Liberation Tigers of Tamil Eelam (LTTE) and could "jeopardize any possibility for future talks.
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Peace broker Norway has been trying to get both sides to the negotiating table, but talks to uphold a ceasefire scheduled for Monday were put off indefinitely by the LTTE citing logistical and security considerations.
Sri Lanka's military blamed a woman from the LTTE for Tuesday's suicide bombing which killed 10 guards and seriously wounded army chief Sarath Fonseka and 30 others.
"A powerful blast activated by a woman Tamil Tiger suicide bomber claimed the lives of several army and civil personnel near the military hospital gate this afternoon," the army said in a statement.
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