US presidents since the 1990s have sought a deeper partnership with India, saying the world's two largest democracies share common values.
But the new charges put India in the same league as Russia, Iran and Saudi Arabia which have faced US opprobrium for alleged transnational attacks.
"I think what we see is that the Modi government has become quite emboldened. This was such a brazen plot," said Lisa Curtis, a supporter of closer US-India ties who coordinated South Asia policy at the White House under former president Donald Trump.
"It sort of fits in line with other things that we've seen happening with the Modi government in terms of human rights, press freedoms and even its stance on Russia," said Curtis, now a senior fellow at the Center for a New American Security.
Journalists, activists and religious minorities have complained of harassment since the Hindu nationalist Modi took power in 2014, and India has rebuffed US calls to end its historic relationship with Russia over its invasion of Ukraine.
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