Speaking in Warsaw after a meeting with his Polish counterpart Donald Tusk, Harper stressed no decisions had been taken, but clearly signalled Canada is ready to do more.
Harper announced a modest new military commitment to the region and acknowledged U.S. President Barack Obama's announcement a day earlier in Warsaw to bolster U.S. military contributions to the area, and to seek $1 billion from Congress.
Poland has been pushing hard for an increased NATO presence in Eastern Europe in the face of Russian aggression in Ukraine.
Obama and Harper are on parallel travels through Europe this week designed to bolster the confidence of Russia's nervous neighbours. Ukraine is not a NATO member, but Poland is, and it is pushing hard for the 28-member military alliance to deploy a more robust military deterrent to the region.
Harper announced Wednesday that Canada will bolster its military forces in the region by deploying a contingent of approximately 75 soldiers for a training exercise to Latvia, who will join the 45 paratroopers already on training manoeuvres in Poland.
The Latvian-bound troops will participate in a U.S.-led exercise dubbed Saber Strike 2014. The Canadian troops will join 2,000 others from nine countries for exercises set for June 9-21 in Poland, Latvia, Estonia and Lithuania.
"We are very much looking at options for additional presence going forward but no decisions have been taken," Harper said.
Harper said the actions of Russian President Vladimir Putin pose a long-term threat to the region that won't disappear any time soon.
"I do believe that what is occurring in Russia under President Putin is a serious development with serious long-term consequences that I don't believe we should think for a moment will disappear in the near future," Harper said.
Harper's announcement, coming on the heels of Obama's Tuesday pledge, was warmly welcomed by Tusk, who made it clear it was about time NATO started focusing eastward.
The Polish prime minister said the commitments were "a harbinger of a certain deep correction in terms of what is of key importance to European security," singling out Canada and the United States.
"These are all very strong signs that are announced, perhaps not a breakthrough because we are allies and we've been allies for many years," said Tusk.
"It's not just the provisions of the treaty ... it's not just symbols and words of support but also a bigger-ever-than-before presence of both the assets, equipment and personnel."
Poland said Tuesday that it would increase its defence spending to two per cent of GDP, to meet the target that NATO has set.
That would make Poland only the sixth NATO country to reach that spending level, well ahead of Canada, where current defence spending stands at one per cent of GDP.
Tusk did not reference that apparent imbalance but said he has discussed the issue with Harper and Obama this week.
Tusk said he would "speak with a strong voice" for more NATO presence in Eastern Europe at this fall's alliance summit in Wales.
Harper has heaped unrelenting criticism on Russian actions in Ukraine, calling Putin a threat to world peace for the annexation of the Crimean Peninsula, and for instigating the continuing unrest in eastern Ukraine.
After meeting with Tusk, Harper said Russia's "aggressive behaviour" required a sustained and co-ordinated response.
But he said any talk of further sanctions against Russia would take place Thursday in Brussels at the G7 leaders' summit.
Harper said it was important to maintain the level of sanctions already in place, and that "pretty clear criteria" must be agreed on before more are put in place.
Harper congratulated Poland on the 25th anniversary of the nation's emergence from communism. He joined Obama and other world leaders in Warsaw later Wednesday in a major celebration to mark the anniversary.
Obama affirmed NATO's Article 5 — that an attack on one member is an attack on all — in a major speech to dozens of world leaders gathered in Warsaw's historic Royal Castle Square, as crowds filled nearby streets.
"These are not just words ... they're unbreakable commitments," Obama said.
The U.S. president also expressed solidarity with non-NATO Ukraine.
"We stand together because we believe that people and nations have the right to determine their own destiny — that includes the people of Ukraine," Obama said.
"Our free nations will stand united so that further Russian provocations will only mean more isolation and costs for Russia," Obama said.
"Because after investing so much blood and treasure to bring Europe together, we refuse to allow the dark tactics of the 20th century to define the 21st."
Harper had a privileged perch at the events, seated between Tusk and one of his political heroes — Lech Walesa, Poland's ex-leader and the founder of its anti-communist Solidarity movement. Wednesday's ceremonies marked Poland's first free elections that brought Solidarity to power.
Obama also met Ukraine's new president-elect, Petro Poroshenko, who was in Warsaw this week for the Polish celebrations.
Harper will also return to Ukraine on Saturday to attend Poroshenko's formal swearing in.
Harper was to depart for Brussels later Thursday. On Friday, he will take the 70th anniversary ceremonies of the Normandy invasion in France.
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