Authorities arrested 26 suspects in a major anti-terror operation spanning several European countries, Belgian officials said Tuesday.

Eleven suspects were arrested after a months-long investigation in Belgium, the Netherlands and Germany. They are suspected of plotting attacks on an unspecified target in Belgium. Officials said they used a jihadist Web site to plan terror attacks.

It was "clear to us that the target was Belgian soil, just not clear enough to say where and when," Belgian public prosecutor Lieve Pellens told CNN.

Police arrested seven people in Antwerp, Belgium, three in Amsterdam, the Netherlands and one near Aachen, in Germany. Those arrested are of Belgian, Dutch, Moroccan and Chechen nationalities, authorities said.

Some of the arrested are suspected of belonging to a Chechen militant group called the Caucasus Emirate. They reportedly tried to raise money and recruit jihadists for the Caucasus Emirate, which aims to create an Islamic emirate in Russia's North Caucasus region of Chechnya, Dagestan and Ingushetia.

In an anti-terror operation that wasn't immediately linked to the first one, Belgian police later Tuesday raided several apartments in Brussels and arrested another 15 terror suspects.

Neither operation is linked to the recent terror warnings in Germany, France and Britain, officials said.

An intelligence source told CNN that U.S. intelligence sources tipped off their Belgian counterparts, which in late 2009 launched the investigation. Pellens said, however, that Belgian authorities were alerted to the group because it used prominent Islamist Web site Ansar al-Mujahideen.

Meanwhile, authorities in Germany were still taking a recent terror alert very seriously.

In a move that has surprised many here, authorities Monday closed off to the public the glass dome of the Reichstag building, one of the top tourist destinations in the city. The Reichstag building, which houses the Parliament, on a busy day draws up to 20,000 visitors.

Germans are on edge, with several false terror alarms in recent days. Heavily armed police have been dispatched to the country's main transport hubs after Interior Minister Thomas de Maiziere last week warned of a heightened risk of an imminent terror attack in the country.

Media reports have said that authorities know about two al-Qaida militants who are in Berlin and plotting commando attacks on people visiting the Reichstag building.