Iran and Syria have announced that they have formed a mutual self-defence pact to confront "threats" now facing them.
The US has withdrawn it ambassador to Syria for what it says are consultations
Iran is under pressure from the US over its nuclear programme, while Syria has come back into sharp focus after the apparent assassination of former Lebanese prime minister Rafik Hariri.
The BBC News website examines the key issues.
Q: Why is Washington concerned about Iran?
The administration suspects Iran is trying to build a nuclear weapon and there are a number of competing views in Washington about what President Bush should do. Some Republican hawks are in favour of taking military action against the Islamic state. There has been much speculation that the US or Israel may try to bomb Iran's nuclear facilities.
On the other hand, Britain, France and Germany have been leading the international effort to negotiate with Iran, and at the beginning of February, President Bush showed support for the negotiations in his State of the Union speech. He also indicated that he would be working for regime change in Iran but not by force.
Iran denies that it has ambitions to build a nuclear bomb and says its nuclear programme is for civilian purposes only.
Q: Why is Washington concerned about Syria?
Syria is the power behind the scenes in neighbouring Lebanon and has some 15,000 troops stationed in the country. The US, supported by the UN Security Council, has for years demanded that those troops be withdrawn.
Last year, the Security Council passed resolution 1559, which called for their withdrawal, and concern has been building in Washington over what it sees as Damascus's foot-dragging in response to the resolution.
Tension increased after former Lebanese Prime Minister Rafik Hariri, who had called for the withdrawal of Syrian troops from the country, was killed in a bomb attack in Beirut on 14 February. Many Lebanese accuse Syria of involvement in the attack, a charge Damascus denies.
The Bush administration also accuses Syria of sponsoring terrorism, by supporting the Palestinian militant groups Hamas and Islamic Jihad and letting Islamic militants enter Iraq from its territory.
It also accuses Syria of backing the Lebanese Shia militant group Hezbollah, which fought Israel's occupation of southern Lebanon and is still involved in border conflicts with Israel.
Q: What is Syria's involvment in Lebanon?
The Syrian troop presence in Lebanon dates back to 1976, when it intervened in Lebanon's civil war to protect the Christian minority against what looked like the imminent victory of radical Palestinians and pan-Arabists. Syria saw that as a threat to its stability. But, having protected the status quo in the civil war, they decided to stay, above all as a means to keep up the pressure in the longer-term conflict with Israel.
Many political leaders in the 1990s accepted the continued Syrian presence as a necessary counter-weight to the Israeli occupation of southern Lebanon. Syrian involvement in its neighbour was formalised by two treaties signed in 1991.
Once the Israelis withdrew from Lebanon in 2000, pressure grew for the Syrians to pull out. Opinion in Lebanon is divided between those who support Syria's presence in their country and those who do not
Q: Are Syria and Iran likely partners?
On the face of it they appear to be strange bedfellows. Iran is an Islamic republic, while Syria is a ostensibly secular Baathist state. They have, however, made common cause against mutual enemies in the past.
In the 1980s, there was much animosity between the two rival Baathist leaders, President Hafez al-Assad and Saddam Hussein of Iraq. Syria was the only Arab country to support Tehran during the Iran-Iraq war of the 1980s.
Syria and Iran also both provide support for Hezbollah.
Q: What are Washington's options?
The US, while not blaming Syria directly for the assassination of Lebanon's former prime minister Rafik Hariri has recalled its ambassador for consultations. This is a common way of displaying diplomatic displeasure.
The US has already imposed a number of sanctions against Syria. Last year, it banned US exports to Syria, apart from food and medicine. It also stopped Syrian aircraft from flying to and from the US and froze the assets of Syrians suspected of violating a law designed to "halt Syrian support for terrorism" passed in 2003.
Reports say that White House officials are now studying the possibility of tougher sanctions on Syria, such as barring investments in the Syrian economy and preventing Syrian banks from using clearing houses in the US.
Analysts say it would be tricky for the US to bomb Iran's nuclear installations. They are thought to be spread around the country and some facilities may be well hidden.