Just a few days ahead of Germany's federal elections, terror warnings have put security forces in Europe's biggest nation on high alert.

On Thursday, police forces holding machine guns were stationed at airports and main train stations across Germany.

Armored vehicles were also deployed on runways including at Frankfurt airport following a warning from the US State Department that al-Qaeda has released a video specifically warning Germany of attacks.

US intelligence monitoring services said of a video message showing a man identified as Abu Talha, warning against Chancellor Angela Merkel's re-election in Sunday's vote.

"Bitter times await the Germans," the militant said in German.

The Berlin government has tightened security to safeguard an election which is widely expected to give Merkel a marked victory, sweeping her into a second term in office.

Merkel enjoys high popularity at home and abroad mostly due to her steady leadership over the last four years and her handling of the country's economic crisis.

The closely watched Ifo index showed business confidence climbed in September for the sixth successive month as Germany slowly untangles out of its deepest post-war recession.

A poll released on Wednesday showed support for the chancellor's conservative Christian Union bloc (CDU/CSU) at 34-35 percent, with the pro-business Free Democrats (FDP) at 12-13 percent.

The Social Democrats (SPD), with Foreign Minister and Vice-Chancellor Frank-Walter Steinmeier as its candidate, scored a meager 26 percent, while the opposition Greens and Left Party were at 11 and 10 percent, respectively.

MRS/AKM