* Tens of thousands of people protested in Homs as monitors toured
* Residents urged Arab observers to see signs of "slaughter"
* United States condemns "escalation of violence"
* Sudanese head of monitoring mission says Homs "reassuring"
By Erika Solomon and Mariam Karouny
BEIRUT, Dec 28 (Reuters) - Arab League monitors aiming to verify whether Syria is ending a military crackdown on protesters said they saw "nothing frightening" in an initial visit to the protest hotbed of Homs although a long investigation would be needed.
Given the brief and limited nature of the monitors' tour on Tuesday, the comments by the chief monitor could heighten the concern of opposition activists that the observer mission could be used as a cloak of respectability by Damascus, issuing assessments whitewashing President Bashar al-Assad's record.
The Syrian Observatory for Human Rights, a British-based activist group, said security forces killed 15 people across the country on Tuesday, six of them in Homs, coinciding with the monitors' visit. Activists said 34 had been killed on Monday.
"Some places looked a bit of a mess but there was nothing frightening," Sudanese General Mustafa Dabi, the chief of the monitoring contingent, told Reuters by telephone from Damascus.
"The situation seemed reassuring so far," he said on Wednesday after his team's short visit to the city of one million people, Syria's third largest and epicentre of nine months of anti-Assad unrest.
"Yesterday was quiet and there were no clashes. We did not see tanks but we did see some armoured vehicles. But remember this was only the first day and it will need investigation. We have 20 people who will be there for a long time."
Activists say about a third of the estimated 5,000 people killed in unrest in Syria since the crackdown began in March died at the hands of security forces in Homs. Dozens have been reported killed in the past week.
The first group of monitors including Dabi were escorted by Syrian authorities into Homs on Tuesday and shown destruction in the restive district of Baba Amr, where Syrian tanks were filmed firing into residential areas the day before, according to amateur video recorded by activists.
Video reports, which cannot be independently verified, have shown parts of Homs looking like a war zone. Constant machinegun and sniper fire is audible and corpses are mangled by blasts. Tanks have been filmed shelling anti-Assad targets in Baba Amr.
The very choice of the Sudanese general to head the League mission has alarmed activists, who say Sudan's own defiance of a war crimes tribunal means the monitors are unlikely to recommend strong action against Assad.
The Arab League says Dabi brings vital military and diplomatic expertise to its unprecedented intervention in the internal crisis of a member state.
But international human rights activists critical of Khartoum say it is all but impossible to imagine a Sudanese general involved in the Darfur conflict ever recommending strong outside intervention, much less an international tribunal, to respond to human rights abuses in a fellow Arab country.
Dabi has held senior Sudanese military and government posts, including in the Darfur region where, the prosecutor of the International Criminal Court says, Sudan's army committed war crimes and the United Nations says 300,000 people may have died.
Sudanese President Omar al-Bashir has been indicted by the Hague-based ICC for genocide and crimes against humanity. Khartoum says the accusations are baseless and politically motivated, and puts the Darfur death toll at 10,000.
WASHINGTON CONDEMNS "INDISCRIMINATE FIRE"
The U.S. State Department condemned what spokesman Mark Toner called an escalation of violence in Homs before the monitors' deployment.
"We have seen horrific pictures of indiscriminate fire, including by heavy tank guns, and heard reports of dozens of deaths, thousands of arrests, as well as beatings of peaceful protesters," Toner said.
"The monitors should have unfettered access to protesters and to areas most severely affected by the regime's crackdown. They bear a heavy responsibility in trying to protect Syrian civilians from the depredations of a murderous regime.
"If the Syrian regime continues to resist and disregard Arab League efforts, the international community will consider other means to protect Syrian civilians," added Toner.
Emboldened by the monitors' first visit, about 70,000 Homs protesters marched towards the city centre on Tuesday where security forces fired shots and teargas at them, activists said.
The military withdrew some tanks shortly before the monitors arrived, in what the activists called a ploy to persuade the monitors that the city was calm. Video on the Internet showed monitors confronted by residents imploring them to venture further into Baba Amr as gunfire crackled around them.
Some fearful residents shouted "We want international protection" at the monitors in a video posted on YouTube.
The monitors were due to make a second tour of Homs on Wednesday. Crowds have pleaded for them to get a thorough look at the most violent neighbourhoods. Tens of thousands of protesters gathered on Tuesday in the Khalidiya district - one of those yet to be visited by monitors.
One activist held up a sign that read: "We are afraid (that)when the monitors leave, they will kill and bury us."
The observers' visit is the first international intervention on the ground in Syria since the uprising began and protesters hope what they witness will prompt world powers to take more decisive action against Assad.
The Syrian leader says he is fighting an insurgency by armed terrorists, and that most of the violence has been aimed at the security forces who have lost some 2,000 men.
International journalists are mostly barred from Syria, making it difficult to confirm accounts from conflict zones.
Activist reports just before the monitors arrived on Tuesday said up to a dozen tanks were seen leaving Baba Amr and others were being hidden to fashion an impression of relative normality in the city while observers were around.
Armed insurgency is eclipsing civilian protest in Syria. Many fear a slide to sectarian war between the Sunni Muslim majority, the driving force of the protest movement, and minorities that have mostly stayed loyal to the government, particularly the Alawite sect to which Assad belongs.
Analysts say the Arab League is anxious to avoid civil war. Western powers have shown no desire to intervene militarily in a volatile region of Middle East conflict. The U.N. Security Council is split, with Russia - a major arms supplier to Assad - and China opposed to any hint of military intervention.
Assad's opponents appear divided on aims and tactics. He still has strong support in important areas, including Damascus and the second city Aleppo, and maintains an anti-Israel alliance with Iran. (Additional reporting by Ayman Samir; Editing by Mark Heinrich)