Law enforcement officials are looking for connections between the 17 terror suspects arrested in Canada and cells in the United States and five other nations.
A court said all 12 adults arrested on Friday and Saturday had been charged with participating in a terrorist group plotting to bomb buildings in Canada. Other charges included importing weapons.
The charges against five minors were not made public. Canadian prosecutors will outline details of their case in a bail hearing on Tuesday.
Authorities said more arrests were possible as police pursue leads about a group that they say was inspired by the violent ideology of the al Qaeda terror network.
"We've by no means finished this investigation," Mike McDonell, deputy commissioner for the Royal Canadian Mounted Police, told AP. "We have a responsibility to follow every lead."
Although Canadian and US officials said over the weekend that there was no indication the purported terror group had targets outside Ontario, McDonell told AP on Monday that there are "foreign connections," but he would not elaborate.
In Washington, a spokesman for the National Security Council at the White House, said the president, George Bush, spoke with Canada's prime minister, Stephen Harper, about the case on Monday afternoon but gave no specifics.
"Prime Minister Harper called the president to update him on the situation involving the arrest of 17 individuals in Toronto who are charged with terrorism-related offenses," spokesman Frederick Jones said.
Responding to the arrests, the US Border Patrol stepped up vigorous inspections of traffic entering the country from Canada and put agents on overtime on high alert along the 6,436km (4,000-mile) border, David Aguilar, the Border Patrol chief, told reporters in Washington on Monday.
A US law enforcement official said investigators were looking for connections between those detained in Canada and suspected Islamic militants held in the United States, Britain, Bangladesh, Bosnia, Denmark and Sweden.
American authorities have established that two men from Georgia who were charged this year in a terrorism case had been in contact with some of the Canadian suspects via computer, the official said, speaking on condition of anonymity because the investigation is continuing.
Prosecutors have said the Georgia men, Ehsanul Islam Sadequee and Syed Haris Ahmed, traveled to Washington to shoot "casing videos" of the Capitol and other potential targets.
A US counterterrorism official said the 17 suspects in Canada are an example of a type of group that authorities have been concerned about for some time: self-organised, ad hoc cells of homegrown extremists, a development first seen in Britain.
The official, also speaking on condition of anonymity, said Canada's government rightfully considered the 17 a serious threat because there was evidence the group was far along in planning attacks.
"It came to a point where our concern for the safety and security of the public far outweighed our appetite for collecting evidence," said McDonell, the RCMP deputy commissioner.
Canadian police say there is no evidence the suspect group had ties to al Qaeda but describe its members as being sympathetic to jihadist ideology. Officials are concerned that many of the 17 suspects were roughly 20 years old and had been radicalised in a short amount of time.
The Ontario Court of Justice released details of the charges faced by the 12 adult men. The men are scheduled to appear in court Tuesday for a bail hearing.
Each is charged with one count of participating in a terrorist group.
Three of them - Fahim Ahmad, 21, Mohammed Dirie, 22, and Yasim Abdi Mohamed, 24 - also are charged with importing weapons and ammunition for the purpose of terrorist activity.
Nine face charges of receiving training from a terrorist group, while four are charged with providing training. Six are charged with intending to cause an explosion that could cause serious bodily harm or death.
No information was released on the five young males arrested due to federal privacy laws that protect minors.
Officials announced Saturday that the suspects were arrested after the group acquired three tons of ammonium nitrate, which can be mixed with fuel oil to make a powerful explosive. One-third that amount was used in the deadly bombing of the Oklahoma City federal building in 1995.
Toronto Mayor David Miller said CN Tower, a downtown landmark, and the city's subway were not targets as had been the speculated in local media, but declined to identify sites that were.