Germany's Foreign Minister Guido Westerwelle has voiced his opposition to foreign military intervention in Syria, telling German Sunday newspaper Bild am Sonntag, "It will only heighten problems and violence" in the country.

Westerwelle also called for Syrian President Bashar Assad to stand trial at the International Criminal Court on charges of war crimes and crimes against humanity.

"The best thing would be for Assad to appear [before the court]," he told the paper. Adding "if one can prevent fresh deaths by Assad's voluntary exit, legal proceedings for me would not have greater importance."

"This would certainly be against my sentiment for justice but the most important thing for me is to stop these deaths and ensure a democratic and peaceful future for Syria," he added.

Clinton talks Syria, in Turkey

US Secretary of State Hillary Clinton discussed Syria's civil war with Turkish leaders in Istanbul on Saturday, one day after Syrian and Jordanian forces exchanged gunfire along their border.

Watch video 01:23 US and Turkey step-up cooperation on Syria

Clinton told reporters during a joint press conference with Turkish Foreign Minister Ahmet Davutoglu that the US and Turkey should begin joint "operational planning" with regard to Syria.

"We have been closely coordinating over the course of this conflict," Clinton said. "But now we need to get into the real details of such operational planning and it needs to be across both of our governments."

"Our intelligence services, our military have very important responsibilities and roles to play so we are going to be setting up a working group to do exactly that," she added.

Clinton also announced that Washington would donate an additional $5.5 million (4.47 million euros) to the United Nations refugee agency to help refugees fleeing the fighting in Syria.

Turkey is currently home to more than 50,000 refugees living in camps along the Syrian border.

Former Syrian ally

Turkey was once a close partner of Damascus but now opposes the Assad regime, which has sought to crush an originally peaceful popular uprising with overwhelming military force.

Watch video 01:46 Rebel groups forced from Salaheddin

Relations between the two countries deteriorated after Syrian fighter jets shot down a Turkish reconnaissance plane in June. Turkey has joined the Arab League in imposing sanctions and travel bans as well as freezing assets of the Assad regime.

Washington imposed a new round of sanctions against Damascus on Friday, blacklisting Syria's state oil company Sytrol for selling gasoline to its close ally, Iran. Tehran is currently struggling with an EU oil embargo and US sanctions, both of which were imposed over the Islamic Republic's disputed nuclear program.

The US Treasury Department also imposed new sanctions against the Lebanese Shiite militia, Hezbollah, for its alleged involvement in Syria's civil war. Washington accuses Hezbollah - also an Iranian ally - of providing training, advice and logistical support to the Syrian government.

Jordanian-Syrian border violence

Clinton's visit to Turkey comes amid heightened tensions in the region, as Syria's civil war threatens to spill over its borders and destabilize neighboring countries.

On Friday, Syrian and Jordanian troops exchanged gunfire across their border. Jordan claims that Syrian forces fired into its territory as they pursued a group of some 500 refugees crossing the border. The Jordanian government says its forces then returned fire, with no casualties reported.

Amman has granted refuge to Syrians fleeing the violence in their homeland, with some 150,000 refugees escaping to Jordan since March 2011. Syrian activists told German news agency dpa that "dozens" of high-ranking military defectors were among the group crossing into Jordan on Friday.

Meanwhile, the head of Germany's foreign intelligence service, Gerhard Schindler, told the daily Die Welt newspaper on Saturday that President Assad's regime could be nearing its end.

"There are a lot of indications that the end phase of the regime has begun," Schindler said, pointing to the trend of defections and desertions from the Syrian military.

jlw, slk/ rc (AFP, dpa, Reuters)