Washington urged the two sides to avoid escalating tensions and said it was touch with both countries.
Indian Prime Minister Manmohan Singh summoned his military chiefs to review New Delhi's "defence preparedness" while his foreign ministry advised Indians not to travel to Pakistan, saying it was unsafe for them to be in the country.
The developments sent ties plummeting to their lowest point since late 2001, when Kashmiri militants staged a brazen attack on the Indian parliament -- an attack New Delhi blamed on the Pakistan-based extremist group Lashkar-e-Taiba.
India has blamed the same group for the Mumbai attacks and has repeatedly said Islamabad is not doing enough to rein in militant groups, a claim that Pakistan rejects.
The nuclear-armed South Asian neighbours -- which have fought three wars since independence from Britain in 1947, two of them over Kashmir -- have said they do not want war this time, but warn they would act if provoked.
Prime Minister Yousuf Raz