STORY: New Delhi laid down a warning on Wednesday to Indian nationals in Canada - and in particular - hundreds of thousands of students:Exercise "utmost caution."The advisory marks another sign of unraveling ties between the two countries in an escalating dispute over the murder of a Sikh separatist leader.Ottawa insists Canada is safe.The travel advisory follows an earlier tit-for-tat expelling of diplomats.JUSTIN TRUDEAU: "...a Canadian on Canadian soil..."Prime Minister Justin Trudeau has said Canada is investigating “credible allegations” that Indian government agents were involved in the June murder of Hardeep Singh Nijjar.India categorically rejects the assertions.It labelled Nijjar a terrorist years ago.On Wednesday, India’s main opposition Congress party endorsed the government's position.SHASHI THAROOR: "We're seeing a tit-for-tat on both sides..."Its leader Shashi Tharoor says Canada's going public has put the countries' relationship in jeopardy: "I think it is unfortunate that Canada chose such a public route." // "Going public, making a statement in parliament and so on was very unfortunate."Meanwhile, in Punjab's holy city of Amritsar some Sikhs called for clarity on the issue - and calm."For a solution they need to sit and talk - basically that's the only way..." said this retired businessman.Protests unfolded among Sikhs in neighboring Pakistan... with protesters calling for India to be held to account.The White House says it's "deeply concerned" about the allegations and is encouraging India to cooperate in any investigation.There’s not much support left for the insurgency in India, but small groups of Sikhs in Australia, Britain, Canada and the U.S. still support the separatist demand.New Delhi has long urged Canada to act against it.As for the killing, Canadian officials have not elaborated on why they think India is involved.