Iran has long been accused of supporting anti-Israeli groups such as Hezbollah in Lebanon and the Palestinian groups, Hamas and Islamic Jihad. But a more recent charge is that it is harbouring members of Osama Bin Laden's al-Qaeda network.
Iran has been accused of supporting Hezbollah, led by Sheikh Nasrallah
Patrick Clawson, an Iran-watcher at the Washington Institute for Near East Policy, an influential think-tank, says the Iranian Government has a contradictory policy towards certain groups.
"Eastern Iran, especially south-eastern Iran, has got a very serious crime problem and large-scale smuggling gangs operate from there," says Mr Clawson.
"The relationships between the central government and the largely Sunni and tribal communities there are terrible. And so Iran has got a problem of controlling that area. It would appear that the Iranian Government is not particularly concerned about the presence of a number of al-Qaeda elements and ex-Taleban elements in that area. That's not a wise move, on their part."
Raising the temperature
Mr Clawson says the Iranian authorities, characteristically, in his view, are partly cracking down, partly turning a blind eye, and partly collaborating with these groups.
Senior Iranian officials have recently admitted holding both senior and junior al-Qaeda members, and say they have deported others.
What has raised the temperature, however, is that US officials have privately alleged that the suicide bombings in May, in the Saudi capital Riyadh, were planned by al-Qaeda officials who are in Iran. If true, that would be a serious accusation.
Iran regards Palestinian groups as freedom fighters
George Perkovich, of the think-tank, the Carnegie Endowment, is not convinced.
"Iran has spent much of the last eight years building relations with Saudi Arabia. Al-Qaeda is the sworn enemy of Iran," he says.
"Furthermore, the Taleban killed Iranian diplomats [during a crisis in Afghan-Iranian relations a few years ago]. The idea that the Iranian Government is somehow going to get in cahoots with its sworn enemy, al-Qaeda, and target a country that it has been improving relations with, Saudi Arabia, doesn't make any sense. I haven't seen evidence marshalled to that effect."
Another recent charge is of Iranian interference in post-war Iraq. Iran has traditional ties to Iraq's majority Shia community, and is suspicious of American intentions there.
Iran and Iraq
"Iran has got a very active and professional propaganda apparatus that's putting in high-quality television and radio programmes into Iraq - and they are malign," says Mr Clawson.
"They spread malicious and inaccurate rumours designed to make US forces look like they're insensitive to Islam, and that's not helpful.
"The actions of the Iranian Government vis-a-vis the (Shia) movements in southern Iraq have been more helpful. But the Iranians are, as is often the case, playing a double game. So in certain areas they're being modestly helpful, and in other areas being profoundly unhelpful."
Iran has kept a close eye on Iraq's Shia community
This, too, is an issue on which there are differing views. Some Washington experts are keeping an open mind about Iran's role in Iraq. The Iranians and the US-led administration in Iraq, they argue, view one another with ambivalence, so that it is too soon to say whether Iran will play a positive or a negative role there.
A third area of debate is over Iran's support for groups which launch attacks against Israel. Here the Iranian Government's track record is much more clear-cut. It has consistently opposed the Middle East peace process, and consistently supported Lebanese and Palestinian groups it regards as freedom fighters rather than terrorists.
But Robert Oakley, a former US ambassador to Pakistan and Somalia, and a veteran expert on counter-terrorism detects a change of behaviour as a result of the US-led war in Iraq.
"I think it has been an intimidating factor. And regimes such as Syria, Iran and others have been much more careful than they might have been otherwise. A small tell-tale sign is to look at the actions of Hezbollah in Lebanon. They have been absolutely quiet as a mouse.
"They haven't lifted a finger to do anything. They have not fired a single Katyusha rocket [at Israel].They have been just as careful as they could be. I assume that's because, first, Hezbollah is a prudent organisation, and it has matured over the years. Second, the Syrians and the Iranians are leaning on them saying, 'Don't do anything because we could be held responsible'."
There's a lively debate in Washington about these and other issues. But there is, nevertheless, a degree of consensus that something needs to be done about Iran.
Read more from Roger Hardy in this series on Iran and the US: