This warning followed a bomb attack in Nairobi's city center

IntelCenter said al-Shabab posted threats on a website Wednesday that said "something big is coming" within the next two weeks. The posting said the country would soon "watch your towers coming down."

"Two weeks from now you will weep," the al-Shabab threat said.

The threat is the third time al-Shabab has said it would target Kenya's tall buildings. The first came days after Kenya sent troops into Somalia last October to fight al-Shabab militants. A second threat came in January.

Al-Shabab is suspected in Monday's bomb attack in central Nairobi that killed one person and wounded more than 30.

The IntelCenter said that the threat of a large attack in Kenya over the next six months is high given the increasing number of threats from al-Shabab.

"The towers most likely to be targeted are those housing hotels, especially those frequented by Westerners, government offices, media and prominent corporations. The building volume of threats and low-level activity indicate that such an attack may be attempted sooner rather than later," the Virginia-based IntelCenter said.

Al-Shabab and al-Qaida formally joined organizations earlier this year, though the ties between the groups were already strong. Al-Shabab counts hundreds of foreign fighters among its ranks, including fighters with experience in the Iraq and Afghanistan wars.

In April the U.S. government warned that it continues to receive information about potential terrorist threats aimed at U.S., Western and Kenyan targets inside Kenya.