Story highlights Assassinations, explosions and computer worms are parts of a covert campaign, analysts say

Iran blames Israel for some covert actions; Israel generally does not comment

Sanctions and economic measures have targeted Iran's central bank, trade and more

On Monday, the EU may implement an embargo on Iranian oil

Iran is facing an extensive and growing international campaign aimed at destabilizing its regime.

The web of pressure includes not only the openly acknowledged moves, such as sanctions, but also mysterious incidents that include assassinations, explosions, and computerized sabotage, according to analysts who follow the country closely.

While the overt actions are spearheaded largely by the United States and allies and quickly encompass more and more countries across the globe, just who is behind the covert activities and why remains a mystery.

In some cases, Iran blames Israel. Israel generally does not comment on such speculation.

U.S. and other Western officials say the goal of the sanctions -- which target Iran's central bank, oil exports, trade deals and more -- is to push the country to cooperate on nuclear issues and sit down at the negotiating table.

Iran insists its nuclear program is for civilian energy purposes; U.S. and other Western officials complain Tehran is seeking a nuclear arsenal.

The mysterious incidents that have struck Iran over the past couple of years involve the country's nuclear and military programs.

Iran calls some of the incidents acts of war.

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Tehran has taken steps of its own, harassing U.S. ships, threatening to impede oil traffic and detaining Westerners -- making them what one analyst calls "potential hostages."

With no sign of serious negotiations on the horizon, both sides have ratcheted up their efforts. On Monday, the European Union could implement an embargo on Iranian oil.

The latest suspicious incident that analysts link to an apparent covert campaign against Iran took place last week.

An attacker on a motorcycle killed an Iranian nuclear scientist, Iranian news outlets reported. An Israeli military spokesman said he had no idea who was behind it, "but I certainly don't shed a tear."

Mickey Segal, a former director of the Iranian department in the Israel Defense Forces' Intelligence Branch, told Israel Army Radio at the time, "Many bad things have been happening to Iran in the recent period. Iran is in a situation where pressure on it is mounting, and the latest assassination joins the pressure that the Iranian regime is facing."

U.S. Secretary of State Hillary Clinton and Defense Secretary Leon Panetta were adamant that the United States had no role in last week's killing.

It was the third time in two years that someone killed an Iranian nuclear scientist by attaching a magnetic bomb to his vehicle.

Analysts who follow Iran closely said they believe Israel, with help from opposition forces in Iran, was most likely behind the assassinations.

The killings came amid reports of mysterious explosions at key Iranian facilities, such as one in November that took place in Isfahan, where Iran has a major nuclear site.

Two weeks earlier, an explosion caused extensive damage at an Iranian military compound, according to the Institute for Science and International Security. At least 17 people, including a general, died in the explosion, Iran state media reported. Two senior U.S. defense officials, speaking on condition of anonymity, said the United States believed that blast was an accident.

Those and other explosions at military facilities in Iran seem "a little bit too coincidental to be a coincidence," said Michael Rubin, resident scholar with the American Enterprise Institute.

Mark Hibbs, senior associate with the Carnegie Endowment for International Peace, agrees.

"Some of the evidence indicates that they're being pinpointed," he said. "It doesn't look like an explosion of some ordnance or explosive material that caused indiscriminate, widespread damage. Instead, some of the damage looks as if it was targeted at parts of an installation."

In May 2011, a blast took place at an oil refinery that President Mahmoud Ahmadinejad was visiting. He was not hurt.

The year before, Iran's nuclear facilities were targeted by a highly specialized computer worm called Stuxnet. It managed to effectively hijack the Natanz nuclear facility and get the centrifuges to destroy themselves, researchers say.

Iranian officials pointed fingers at Israel and the United States; officials from neither country commented.

More recently, the crash of an American drone in Iran revealed covert U.S. activity in that country.

The Sentinel drone was on a mission to survey suspected nuclear sites in the country, U.S. military officials told CNN.

In public comments, U.S. officials generally focus on the sanctions, including those imposed by the United States and the U.N. Security Council, which U.S. officials call "unprecedented" for Iran.

For example, Iranian planes have been grounded when countries refused to refuel the planes due to the sanctions.

The logic behind the sanctions is "that at some point the pain for Iran will be too great and it will give in," Hibbs said. "So far, that hasn't happened."

Iran, meanwhile, has struck back on numerous fronts.

Recently, it threatened to close the Strait of Hormuz, a critical transit point for one-fifth of the world's oil. The United States warned Iran against such a move.

U.S. military and Coast Guard ships had two close encounters this month with high-speed Iranian boats that exhibited provocative behavior in the Strait of Hormuz and Persian Gulf, U.S. military officials said.

While many analysts, including Hibbs, refer to the threat to close the strait as "saber-rattling," Trita Parsi, president of the National Iranian-American Council, says that if Iran considers itself as being engaged in a war, it might actually try such a move.

"That's part of the reason they're playing up the Strait of Hormuz card," said Parsi, author of the book "A Single Roll of the Dice: Obama's Diplomacy with Iran."

"It's extremely counterproductive and would inflict a tremendous amount of damage on Iran itself. But in wartime, you could do things like that."

Iranian Oil Minister Rostam Ghasemi recently complained that Iran's enemies are waging "economic war," according to state-run news agency IRNA. And state-run PressTV quoted a member of the country's committee on national security and foreign policy, Heshmatollah Falahatpisheh, as saying last month that the violation of Iran's airspace by a U.S. spy drone amounts to an "act of war."

Analysts also point to other steps Iran is taking as likely responses to the increasing pressure.

Among them are the arrests of Westerners. Most recently, Iran sentenced Amir Mirzaei Hekmati, a U.S. ex-Marine, to death for alleged espionage. His family and the U.S. government deny the accusations.

Hekmati and others are "potentially pawns in a game that Iran can play with the West," Hibbs said. "Foreigners are potential hostages in Iran to a negotiation."

Analysts also note the November siege on the British Embassy in Tehran, in which protesters stormed the embassy and a separate diplomatic compound. The British government warned Iran of "serious consequences" as a result.

The incident was seen as Iran, "with informal supporters that it orchestrates," threatening the safety of diplomats from "P5+1" countries, Hibbs said. P5+1 refers to the five permanent members of the U.N. Security Council -- China, France, Russia, the United Kingdom and the United States -- along with Germany. They have been working to bring Iran to the negotiating table.

Finally, analysts say Iran's activities in Iraq and Afghanistan are seen as a response to the pressure being mounted against it.

While the full extent of Iran's activities in each country is unclear, U.S. officials have spoken out against the idea of Iran trying to build its influence.

U.S. officials said last year there was strong evidence that Shiite militias in Iraq were using Iranian weapons to attack U.S. troops. In October, Clinton warned Iran not to view the withdrawal of U.S. troops from Iraq as an opportunity to increase its role.

"No one, most particularly Iran, should miscalculate about our continuing commitment to and with the Iraqis going forward," she said.

President Ahmadinejad said his country will not increase its involvement with Iraq because of the U.S. withdrawal. "I don't think there is going to be any change," he said at the time.

Last year, the United States accused Iran of providing sanctuary to an al Qaeda network that provides help to jihadists moving between the Middle East, Afghanistan, and Pakistan.

In December, the Congressional Research Service said in a report that the Obama administration "identifies Iran as a major threat to U.S. national security interests" based not only on its nuclear program but also its "support for militant groups in the Middle East and in Iraq and Afghanistan."

As the web of pressure over Iran continues to grow, meanwhile, it is unclear what could lie ahead.

Panetta said Wednesday that diplomacy is "always an option" to pursue.

"And we've always expressed a willingness to try to do that," he said. "But we've always made clear that in terms of any threats to the region, in terms of some of the behavior that they've conducted in the region, that we'll also be prepared to respond militarily if we have to."