Diplomatic relations between Canada and the Soviet Union took another turn for the worse with Ottawa's expulsion and banning of two more Soviet diplomats and a warning from Moscow that it will retaliate.

Canada's expulsion Thursday of a Soviet diplomat and the banning of a former second secretary at the Soviet Embassy in Ottawa brought to 19 the number of Russian officials ordered out of the country since last week.Ottawa's latest actions were in retaliation for Moscow's announcement late Wednesday that it was expelling two Canadian diplomats and banning three others from entering the country.

Against a backdrop of easing U.S.-Soviet tensions, Canada set off the "expulsion war" last week by expelling eight Soviet diplomats and banning nine other officials from returning to the country after a massive investigation by Canada's spy agency discovered they were involved in espionage operations.

Col. Grigori Stepanovich Roublev, the military attache in the Soviet Embassy in Ottawa, was expelled Thursday. Sergey Mikhaylovich Kashtanov, a former second secretary at the embassy, was declared persona non grata.

At the same time, the government cut by three the total number of Soviet officials that can be assigned to Canada.

In Moscow, the Soviet Union warned earlier it was ready to expel more Canadian diplomats if the Ottawa government took any further action against Soviet representatives.

The threat was contained in a diplomatic note delivered Wednesday to the Canadian Embassy in which the Soviet government said it was expelling two Canadian diplomats and barring three others from entering the country again, the official news agency Tass said.

"The Soviet side hopes that the Canadian side will display a sense of responsibility and realism and refrain from further attempts at preventing the normal functioning of the Soviet representations in Canada," said the Soviet note, delivered Wednesday night by Deputy Soviet Foreign Minister Alexander Bessmertnikh to Ambassador Vernon G. Turner, according to Tass.

"If, contrary to this warning, it further aggravates the situation, it shall expect immediate additional countermeasures by the Soviet side."

The two Canadian diplomats ordered out of the country within three days were Naval Attache Cmdr. Raymond Steele and Press Attache Evelyn Puxley, a second secretary.

The Soviet protest said the two Canadians had "engaged themselves in activities which are absolutely unacceptable" - diplomatic jargon for spying.

Puxley said she would continue at her job as embassy spokeswoman until she left Friday.

"I am sorry. I would love to say more but you can imagine how I feel personally about this unjustified expulsion," she said.