NEAR SHATT AL-ARAB Mandated to secure Iraqi waters, the commander of a U.S. gunboat orders his crew to watch an Iranian look-out post for activity as his ship sails through waters with no clear divide between Iraq and Iran.
It was in this murky stretch that 15 British military personnel were captured by Iran earlier this year and accused of straying into its waters. A long-running dispute over the nearby Shatt al-Arab border river has helped spark war in the past.
Now, the U.S. military and its allies patrol as close as possible to where they believe Iraq's territorial waters end, but as tension rises between the Islamic Republic and the West, so does the potential for accidental escalation.
"We set a line where we think it's reasonable, and by customary use, going up and down that line, we try and set the tone," said John Chandler of the Australian HMAS Anzac warship, part of the coalition patrolling Iraqi waters.
"The Iranians have slightly different view as to where the line is."
The last time Iran and Iraq agreed on the division of their waterways was in 1975, when the Shatt al-Arab river and its mouth into the Gulf were split along their deepest channel.
Since then, Western navy sources said, silt has shifted the riverbed's topography but no new marine border has been defined, leaving right of passage to custom: use it or lose it.
According to a translator aboard a U.S. warship patrolling the disputed waters, close to the mouth of the Shatt al-Arab and Iraq's oil terminals, the Iranian navy is not happy.
"I won't translate it exactly, but sometimes the Iranians swear at the Americans, curse their parents and call me a conspirator," said the translator, who declined to be named.
The dispute over the border river, which Iran calls Arvand Rud, was a main cause of the 1980-1988 Iran-Iraq war.
In 2004, Iran seized and later released eight British naval personnel it accused of straying into its waters. In March it grabbed another 15 British sailors for the same alleged offence.
The sailors in the most recent incident, who were also freed, denied trespassing in Iranian waters, but a British parliamentary report last week criticized the use of a map showing a defined boundary, which experts said does not exist.
USS Typhoon Commander Joel Lang does not let the incident deter him from taking his gunboat into the troubled waters, in a bid to back Iraq's territorial claims with military muscle.
"We're going to drive down the line on the Iraqi side, and we're going to drive down slow. For Americans, we see this as driving down the strip on a Friday night, waving to all our friends," he said on the way to the disputed area.
Lang said his ship was "armed to the teeth", but he takes care to keep his gun teams discreet to avoid provocation. He likened his role to an armed policeman patrolling a beat.
"We may get to wave at some Iranians and some of their gunboats as we go down. Sometimes they wave back. Sometimes they tell us we're number one," he said, referring to the extension of the middle finger, an obscene gesture in Western society.
Relations between Iran and the West are already strained, as the West accuses Tehran of trying to build nuclear weapons. Iran says its nuclear goals are limited to generating electricity.
The countless bullet and missile holes in Iraq's Khor al-Amaya oil terminal are a reminder of the Iran-Iraq war. U.S.-led and Iraqi forces are determined not to let the oil platforms fall victim to regional tensions again.
The terminal is one of two Iraqi platforms through which most of the country's oil -- the world's third largest reserves -- is pumped, and on which its battered economy relies.
Iraqi marines guarding the platform accuse neighboring states of stoking insurgent attacks in Iraq. Other Iraqi marines, tasked with searching vessels nearby, say their relationship with Iranian mariners in particular is not good.
Searching an Iranian vessel requires special clearance and more bureaucratic checks than boarding vessels of other nationalities, a U.S. navy official said.
Three km (1.86 mile) exclusion zones around the oil terminals have been in place since 2004, when suicide bombers in boats attacked Khor al-Amaya's sister al-Basra oil terminal.
On the edge of the Khor al-Amaya exclusion zone, visible from the oil terminal, the Iranian look-out post keeps watch.
The U.S. Navy says the armed post is built on a sunken Iraqi crane, which Iran says is in its waters. The neck, on which the post is constructed, leans out over Iraqi waters, the U.S. navy says. Iran probably disagrees.